The first programming language for the Xerox Alto was BCPL, the language that led to C.
This article shows how to write a BCPL "Hello World" program using Bravo, the first WYSIWYG text editor, and run it on the Alto simulator.
The Xerox Alto is the legendary minicomputer from 1973 that helped set the direction for personal computing.
Since I'm helping restore a Xerox Alto (details), I wanted to learn more about BCPL programming.
(The influential Mesa and and Smalltalk languages were developed on the Alto, but those are a topic for another post.)
The Xerox Alto II XM computer. Note the video screen is arranged in portrait mode. Next to the keyboard is a mouse. The Diablo disk drive is below the keyboard. The base contains the circuit boards and power supplies.
Using the simulator
Since the Alto I'm restoring isn't running yet, I ran my BCPL program on Salto, an Alto simulator for Linux written by Juergen Buchmueller.
To build it, download the simulator source from
github.com/brainsqueezer/salto_simulator,
install the dependencies listed in the README,
and run make.
Then run the simulator with the appropriate Alto disk image:
bin/salto disks/tdisk4.dsk.Z
Here's what the simulator looks like when it's running:
The Salto simulator for the Xerox Alto.
(To keep this focused, I'm not going to describe everything you can run on the simulator,
but I'll point out that pressing ? at the command line will show the directory contents.
Anything ending in .run is a program you can run, e.g. "pinball".)
Type bravo to start the Bravo text editor.
Press i (for insert).
Enter the BCPL program:
// Hello world demo
get "streams.d"
external
[
Ws
]
let Main() be
[
Ws("Hello World!*N")
]
Here's a screenshot of the Bravo editor with the program entered:
A Xerox Alto 'Hello World' program for written in BCPL, in the Bravo editor.
Press ESC to exit insert mode.
Press p (put) to save the file.
Type hello.bcpl (the file name) and press ESC (not enter!).
Press q then ENTER to quit the editor.
Run the BCPL compiler, the linker, and the executable by entering the following commands at the prompt:
bcpl hello.bcpl
bldr/d/l/v hello
hello
If all goes well, the program will print "Hello World!"
Congratulations, you've run a BCPL program.
Output of the Hello World program in BCPL on the Xerox Alto simulator.
The following figure explains the Hello World program.
If you know C, the program should be comprehensible.
'Hello World' program in BCPL with explanation.
The BCPL language
The BCPL language is interesting because it was the grandparent of C.
BCPL (Basic Combined Programming Language)
was developed in 1966. The B language was developed in 1969 as a stripped down version of BCPL by Ken Thompson and Dennis Ritchie. With the introduction of the PDP-11, system software needed multiple datatypes, resulting in the development of the C language around 1972.
Overall, BCPL is like a primitive version of C with weirdly different syntax.
The only type that BCPL supports is the 16-bit word, so it doesn't use type declarations.
BCPL does support
supports C-like structs and unions, including structs that can access
bit fields from a word. (This is very useful for the low-level systems programming tasks that BCPL was designed for.)
BCPL also has blocks and scoping rules like C, pointers along with lvalues and rvalues, and C-like looping constructs.
A BCPL program looks strange to a C programmer because many of the special characters are different and BCPL often uses words instead of special characters. Here are some differences:
Blocks are defined with [...] rather than {...}.
For array indexing, BCPL uses a!b instead of a[b].
BCPL uses resultis 42 instead of return 42.
Semicolons are optional, kind of like JavaScript.
For pointers, BCPL uses lv and rv (lvalue and rvalue) instead of & and *.
rvalues.
The BCPL operator => (known as "heffalump"; I'm not making this up) is used for indirect structure references instead of C's ->.
selecton X into, instead of C's switch, but cases are very similar with fall-throughs and default.
lshift and rshift instead of << and >>.
eq, ne, ls, le, gr, ge in place of ==, !=, <, <=, >, >=.
test / ifso / ifnot instead of if / else.
A BCPL reference manual is here if you want all the details of BCPL.
More about the Bravo editor
The Bravo editor was the first editor with WYSIWYG (what you see is what you get)
editing. You could format text on the screen and print the
text on a laser printer.
Bravo was written by Butler Lampson and Charles Simonyi in 1974. Simonyi later moved to Microsoft, where he wrote Word, based on the ideas in Bravo.
Steve Jobs saw the Alto when he famously toured Xerox Parc in 1979, and it inspired the GUI for the Lisa and Mac.
However, Steve Jobs said in a commencement address,
"[The Mac] was the first computer with beautiful typography. If I had never dropped in on that [calligraphy] course in college, the Mac would have never had multiple typefaces or proportionally spaced fonts. And since Windows just copied the Mac, it's likely that no personal computer would have them."
This is absurd since the Alto had a variety of high-quality proportionally spaced fonts in 1974, before the Apple I was created, let alone the Macintosh.
The image below shows the Hello World program with multiple fonts and centering applied.
Since the compiler ignores any formatting, the program runs as before.
(Obviously styling is more useful for documents than code.)
The Bravo editor provides WYSIWYG formatting of text.
The manual for Bravo is here but
I'll give a quick guide to Bravo if you want to try more editing.
Bravo is a mouse-based editor, so use the mouse to select the text for editing.
left click and right click under text to select it with an underline.
The editor displays the current command at the top of the editing window.
If you mistype a command, pressing DEL (delete) will usually get you
out of it.
Pressing u provides an undo.
To delete selected text, press d.
To insert more text, press i, enter the text, then ESC to exit insert mode.
To edit an existing file, start Bravo from the command line,
press g (for get), then enter the filename and press ESC.
To apply formatting, select characters, press l (look), and then enter a formatting code
(0-9 to change font, b for bold, i for italics).
Troubleshooting
If your program has an error, compilation will fail with an error message. The messages don't make much sense, so try to avoid typos.
The simulator has a few bugs and tends to start failing after a few
minutes with errors in the simulated disk.
This will drop you into the Alto debugger, called Swat.
At that point, you should restart the simulator. Unfortunately any files you created in the simulator will be lost when you exit the simulator.
If something goes wrong, you'll end up in Swat, the Xerox Alto's debugging system.
Conclusion
The BCPL language (which predates the Alto) had a huge influence on programming languages since it led to C (and thus C++, Java, C#, and so forth).
The Bravo editor was the first WYSIWYG text editor and strongly influenced personal computer word processing.
Using the Alto simulator, you can try both BCPL and Bravo for yourself by compiling a "Hello World" program, and experience a slice of 1970s computing history.
How does a tiny chip time the runners in the Bay to Breakers race? In this article, I take die photos of the RFID chip used to track athletes during the race.
Bay to Breakers is the iconic San Francisco race, with tens of thousands of runners (many in costume and some in nothing) running 12km across the city.
To determine their race time, each runner wears an identification bib.
As you can see below, the back of the bib has a small foam rectangle with a metal foil antenna and a tiny chip underneath. The runners are tracked using a technology called RFID (Radio Frequency Identification).
The bib worn by runners in the Bay to Breakers race. At the top, behind the foam is an antenna and RFID chip used to detect the runner at the start and end of the race.
At the beginning and end of the race, the runners cross special mats that
contain antennas and broadcast ultra high frequency radio signals.
The runner's RFID chip detects this signal and sends back the athlete's ID number, which is programmed into the chip.
By tracking these ID numbers, the system determines the time each runner took to run the race.
The cool thing about these RFID chips is they are powered by the received radio signal; they don't need a battery.
Mylaps, whose name appears on the foam rectangle, is a company that supplies sports timing systems: the bibs with embedded RFID chips, the detection mats, and portable detection hardware.
The detection system is designed to handle large numbers of runners, scanning more than 50 tags per second.
Removing the foam reveals an unusually-shaped metal antenna, the tiny RFID chip (the black dot above the word "DO", and the barely-visible word "Smartrac".
Studying the Smartrac website reveals that this chip is the
Impinj Monza 4 RFID chip, which operates in the 860-960 MHz frequency range and is recommended for sports timing.
The RFID circuit used to detect runners in the Bay to Breakers. The metal forms an antenna. The tiny black square in the center is the RFID chip.
Getting the chip off the bib was a bit tricky. I softened the bib material in Goof Off, dissolved the aluminum antenna metal with HCl and removed the adhesive with a mysterious BGA adhesive solvent I ordered from Shenzhen.
The chip itself is remarkably tiny, about the size of a grain of salt. The picture below shows the chip on a penny, as seen through a microscope: for scale, a grain of salt is by the R and the chip is on the U (in TRUST).
This is regular salt, by the way, not coarse sea salt or kosher salt.
I spent a lot of time trying to find the chip when it fell on my desk, since it is practically a speck.
The RFID chip used to identify runners is very small, about the size of one of the letters on a penny. A grain of salt (next to R) and the RFID chip (next to U).
In the picture above, you can see the four round contact points where the chip was connected to the antenna.
There's still a blob of epoxy or something around the die, making it hard to see the details.
The chip decapsulation gurus use
use boiling nitric and sulfuric acids to remove epoxy, but I'm not that hardcore so I heated the chip over a stove flame. This burned off the epoxy much better than I expected, making the die clearly visible as you can see in the next photo.
I took 34 die photos using my metallurgical microscope and stitched them together to get a hi-res photo. (I described the stitching process in detail here). The result is the die photo below (click it for the large image).
Surprisingly, there is no identifying name or number on the chip.
However, comparing my die photo with the picture in the
datasheet confirms that the chip is the Monza 4 RFID chip.
I can identify some of the chip's layout, but the chip is too dense and has too many layers for me to reverse engineer the exact details.
Thus, the description that follows is slightly speculative.
Die photo of the Impinj Monza 4 RFID chip.
The four pins in the corners are where the antenna is connected. (The chip has four pins because two antennas can be used for improved detection.)
The left part of the chip is the analog logic, extracting power from the antenna, reading the transmitted signal, and modulating the return signal.
The rectangles on the left are probably transistors and capacitors forming a charge pump to extract power from the radio signal (see patent 7,561,866).
The right third of the chip is so-called "random logic" that carries out the commands sent to the chip. According to the datasheet, the chip uses a digital logic finite state machine, so the chip probably doesn't have a full processor.
The 32 orderly stripes in the middle are the non-volatile memory (NVM).
Above the stripes, the address decode circuitry is barely visible. The chip has 928 bits of storage (counting up the memory banks on the datasheet) so I suspect the memory is set up as a 32x29 array.
Some NVM details are in patent 7307534.
Along the lower and right edges of the chip, red lines are visible; these connect chips together on the wafer for testing during manufacturing (patent 7307528).
The Impinj Monza 4 RFID chip on top of a 8751 microcontroller chip shows that the RFID chip is very small and dense.
To show how small the chip is, and how technology has changed, I put the RFID chip on top of an 8751 microcontroller die. The 8751 microcontroller is a chip in Intel's popular 8051 family dating from 1983. Note that the circuitry on the RFID chip is denser and the chip is much, much smaller.
(The photo looks a bit photoshopped, but it genuinely is the RFID chip sitting on the surface of the 8751 die. I don't know why the RFID chip is pink.)
So, if you ran in the Bay to Breakers, that's the chip that tracked your time during the race.
(There aren't a lot of other RFID die photos on the web, but a few are at
Bunnie Studios,
Zeptobars and ExtremeTech if you want to see more.)
A few days ago, I wrote about how I'm helping restore a Xerox Alto for Y Combinator.
This new post describes the first day of restoration: how we disassembled the computer and disk drive and fixed a power supply problem, but ran into a showstopper problem with the disk interface.
The Xerox Alto was a revolutionary computer from 1973, designed by computer pioneer Chuck Thacker at Xerox PARC
to investigate ideas for personal computing.
The Alto was the first computer built around a mouse and GUI, as well as introducing Ethernet and laser printers to the world.
The Alto famously inspired Steve Jobs, who used many of its ideas in the Lisa and Macintosh computer.
Alan Kay, whose vision for a personal computer guided the Alto, recently gave an Alto computer to Y Combinator.
Getting this system running again is a big effort but fortunately I'm working with a strong team, largely from the IBM 1401 restoration team. Marc Verdiell, Luca Severini, Ron, Carl Claunch, and I started on restoration a few days ago, as shown in Marc's video below.
Disassembling the Alto
We started by disassembling the computer.
The Xerox Alto has a metal cabinet about the size of a dorm mini-fridge, with a Diablo hard disk drive on top, and a chassis with power supplies and the circuit boards below. With some tugging, the chassis slides out of the cabinet on rails as you can see in the photo below. At the front are the four cooling fans, normally protected by a decorative panel.
Note the unusual portrait layout of the display.
The Xerox Alto II XM 'personal computer'. The card cage below the disk drive has been partially removed. Four cooling fans are visible at the front of it.
With the chassis fully removed, you can see the four switching power supplies on the left, the blue metal boxes. The computer's circuit boards are on the right, not visible in this picture.
The wiring for the backplane is visible at right front, with pins connected by wire-wrapped wire connections. This wiring connects the circuit boards together.[1]
The Alto's chassis has been removed. On the left are the four switching power supplies (blue boxes). On the right, the connections for the wire-wrapped backplane are visible. The circuit boards plug into this backplane.
The power supplies
Our first goal was to make sure the power supplies worked after decades of sitting idle.
The Alto uses high-efficiency switching power supplies.[2]
To explain the power supplies in brief, input power is chopped up thousands of times a second to produce a regulated voltage. Unlike modern computer power supplies, there's a second switching stage (the inverter), which drops the voltage to the desired 15 volts.
This was more complexity than I expected, but
fortunately the detailed power supply manual was available online, thanks to Al Kossow's bitsavers.[3]
We tested each power supply with a resistor as a dummy load and checked that the output voltage was correct. We also used an oscilloscope to make sure the output was stable.
All the power supplies worked fine, except for the +15V supply (top center), which had trouble getting up to 15 volts and staying there.
We disassembled the faulty power supply to track down the problem.
The photo of the power supply below shows how densely components are crammed into the power supply. Two of the circuit boards have been removed and are at the back. Note the three large filter capacitors at the front.
Switching power supply from the Xerox Alto computer. Two of the control boards have been removed and are visible at back.
We noted signs of overheating on the AC connector, as well as a somewhat sketchy looking repair (a trace replaced by a wire) and some signs of corrosion. Apparently the power supply had problems in the past and had been serviced. We cleaned up the corrosion and it appeared to be superficial.
The power supply disassembled easily for repair, as you can see below.
The main board is at the right.
The tower of three inductors on the main board is an unusual way of mounting inductors.
Three circuit boards (top) plug into the main board. Because the power supply uses discrete components instead of a modern SMPS control IC, it needs a lot of control circuitry.
The switching transistors (lower center) are mounted onto metal heat sinks for cooling.
The Alto's switching power supply, disassembled. The main board is in the lower right. The three circuit boards are at top, below the large input capacitors.
The large capacitors were attached with screws, making it easy to remove them for testing.
A capacitance meter showed that the three large capacitors had failed, explaining why the power supply had trouble outputting the desired voltage.
Ron went off and found replacement capacitors, although they weren't an exact match for the originals. With the new capacitors mounted in place, the power supply worked properly.
Inside the Diablo disk drive
We also looked at the Diablo disk drive, which provides 2.5MB of storage for the Xerox Alto. The first step was removing the disk pack.
In normal operation, the front of the drive is locked shut to keep the disk from being removed during use.
To remove the disk without powering up the drive, we had to open the drive and manually trip the latch that locks it shut (see
Diablo drive manual).
This picture shows the disk pack being reinserted into the drive.
Unlike modern hard disk drives, the Alto's disk can be removed from the drive.
Users typically used different disks for different tasks — a programming disk, a word processing disk, and so forth.
The disk pack is a fairly large white package, resembling a cross between an overgrown Frisbee and a poorly-detailed Star Wars spaceship.
The drive's multiple circuit boards are also visible in the photo.[4]
Inserting a 2.5 MB hard disk pack into the Diablo drive used by the Xerox Alto computer.
As the disk pack enters the drive, it opens up to provide access to the disk surface.
The photo below shows the exposed surface of the disk, brownish from the magnetizable iron oxide layer over the aluminum platter.
The read/write head is visible above the disk's surface, with another head below the disk. The disk stores data in 203 concentric pairs of tracks, with the heads moving in and out together to access each pair of tracks.
Closeup of the hard disk inside the Diablo drive. The read/write head (metal/yellow) is visible above the disk surface (brown).
Although the heads are widely separated during disk pack insertion, they move very close to the disk surface during operation,
floating about one thousandth of a millimeter above the surface.
The diagram below from the manual helps visualize this minute distance, and illustrates the danger of particles on the disk's surface.
The Diablo disk and why contaminants are bad, from the Alto disk manual.
The disk interface cliffhanger
The final activity of the day was making sure all the Alto's circuit boards were in the right slots and the cables were all hooked up properly.[5]
Everything went smoothly until I tried to hook up the Diablo disk drive to the disk interface card: the disk drive cable didn't fit on the card's connector!
The cable to the Alto disk didn't fit onto the disk interface card!
After trying various combinations of cables and edge connectors,
we discovered that the rainbow-colored ribbon cable you can see in the lower right above did fit the disk interface card. But instead of going to the Diablo disk drive, this cable went to a connector on the back of the Alto labeled "Tricon".
Tricon is the controller for the Trident Disk, a high-capacity disk drive that could be used with the Alto, providing 80 MB instead of the just 2.5 MB that the standard Diablo drive provides.
Looking at the disk interface card more closely, we saw it was labeled "Alto II Trident Disk Interface" (upper left corner of the photo below), confirming that it was for the Trident.
Trident Disk Interface card for Xerox Alto computer. (See label in upper left.)
It was a shock to discover the disk interface card was for the Trident drive, since our Alto has
the standard Diablo drive, which is completely incompatible with the Trident.[6]
We checked all the boards and verified that the system was missing the Diablo interface board.
This was a showstopper problem; with the wrong board, the disk drive would be unusable and we wouldn't be able to boot up the system.
What could we do? Network boot the Alto? Build a disk simulator? Find a Trident drive on eBay? (We actually found a Trident disk platter on eBay for $129, but no drive.)
Tune in next episode to find out what we did about the disk interface problem. (Spoiler: we found a solution thanks to Al Kossow.)
Notes and references
[1]
The physical layout of the power supplies is specified on page 11 of
the Alto documentation introduction.
On the top are three Raytheon/Sorensen power supplies, +12V (15A), +15V (12A), and -15V (12A).
At the bottom is a large LH Research Mighty Mite power supply providing +5V (60A) and -5V (12A).
Why the variety of voltages? Most of the circuitry in the Alto uses 5V, which is standard for TTL chips.
The MOS memory chips use +5V, -5V and +12V. The Ethernet card uses +15V and -15V, with +15V powering the transceiver. The disk drive uses +/- 15V.
[2]
Steve Jobs claimed that the Apple II's use of a switching power supply was a revolutionary idea
ripped off by other computer manufacturers. However, the Alto is just one of many computers that used switching
power supplies before Apple (details).
[3]
For full details on the power supply operation, see the
block diagram.
First, the 115V AC line input is converted to 300V DC by a rectifier and voltage doubler. (The voltage doubler is a clever way of supporting both 115V and 230V inputs; using the doubler with 115V. This is why older PCs have a switch on the power supply to select 115V or 230V. Modern power supplies handle a wide input range, and don't require a switch.)
Next, the power supply has a chopper, a PWM transistor circuit that chops up the 300V DC, producing a regulated 120V-200V DC, depending on the output load. This goes to the inverter, which drives a step-down/isolation transformer that produces the desired 15V output. A regulation circuit sends feedback to the chopper based on the output voltage. Meanwhile, an entirely separate switching power supply circuit generates voltages (including +150V) used by the power supply internally.
Modern power supplies use a single switching stage in place of the separate chopper and inverter. I believe the two stages were used to reduce the load on the bipolar switching transistors, which don't have the performance of modern MOSFET switching transistors. (As Ron pointed out, modern power supplies often have a PFC (power factor correction) stage for improved efficiency. Thus, the two-stage design has returned, although the stages are entirely different now.)
Modern power supplies use a power supply control IC. The Alto's power supply instead has control circuits built from simple components: transistors, op amps, 555 timers.
This is one reason the power supply requires three circuit boards.
[4]
The following table from the Alto disk manual gives the stats for the drive.
Statistics on the Diablo 31 disk used with the Xerox Alto computer.
[5]
The Alto backplane has 21 slots, not all of which are used in our system.
The list of which board goes into which slot is on page 8 of
the Alto documentation.
[6]
I suspect that the Y Combinator Alto originally had both a Trident drive and a Diablo drive (as well as four Orbit boards to drive a laser printer), and when it was taken out of service, the Trident drive, the Diablo interface board, and the Orbit boards went off somewhere else.
This left the Alto with a drive that didn't match the interface card.
For reference, schematics and documentation on the Trident interface board are here.
Despite all the chips on the disk interface board, it doesn't do very much, since each TTL chip is fairly simple.
The interface board has some counters, one word data buffers, parallel/serial conversion, and a bit of control logic.
The Alto was designed to offload many hardware tasks to microcode, so the hard work of disk I/O is performed in microcode (software).
Alan Kay recently gave his 1970's Xerox Alto to Y Combinator and I'm helping with the restoration
of this legendary system.
The Alto was the first computer designed around a graphical user interface
and introduced Ethernet and the laser printer[1]
to the world.
The Alto also was one of the first object-oriented systems, supporting the Mesa and Smalltalk languages.
The Alto was truly revolutionary when it came out in 1973, designed by computer pioneer Chuck Thacker.
Xerox built about 2000 Altos for use in Xerox, universities and research labs, but the Alto was never sold as a product.
Xerox used the ideas from the Alto in
the Xerox Star, which was expensive and only moderately successful.
The biggest impact of the Alto was in 1979 when Steve Jobs famously toured Xerox and saw the Alto and other machines.
When Jobs saw the advanced graphics of the Alto, he was inspired to base the user interfaces of the Lisa and Macintosh systems on Xerox's ideas, making the GUI available to the mass market.[2]
How did Y Combinator end up with a Xerox Alto? Sam Altman, president of Y Combinator has a strong interest in the Alto and its place in computer history. When he mentioned to Alan Kay that it would be fun to see one running, Alan gave him one.
This article gives an overview of the Alto, its impact, and how it was implemented.
Later articles will discuss the restoration process as we fix components that have broken over the decades and get the system running.
The Xerox Alto II XM computer. Note the video screen is arranged in portrait mode. Next to the keyboard is a mouse. The Diablo disk drive is below the keyboard. The base contains the circuit boards and power supplies.
The photo above shows Y Combinator's Alto computer.
The Alto has an unusual portrait-format display, intended to match an 8½" by 11" page of paper.
Most displays of the time were character-oriented, but the Alto had a bitmapped display, with each of the 606x808 pixels controllable independently. This provided unprecedented flexibility for the display and allowed WYSIWYG (what-you-see-is-what-you-get) editing.
The bitmapped display memory used almost half the memory of the original Alto, however
In front of the keyboard is the three-button mouse.
Xerox made the mouse a fundamental input device for the Alto and designed the user interface around the mouse.
The disk drive at the top of the cabinet takes a removable 2.5 megabyte disk cartridge. The small capacity of the disk was a problem for users, but files could also be accessed over the Ethernet from file servers.
The lower part of the cabinet contains the computer's circuit boards and power supplies, which will be discussed below.
Dynabook and the vision of the Alto
The motivation for the Alto was Alan Kay's Dynabook project.
In 1972, he wrote A Personal Computer for Children of All Ages, setting out his vision for a personal, portable computer for education (or business), with access to the world's knowledge.
In effect, Alan Kay presented a detailed vision for the touchscreen tablet decades before
it was practical.[3]
Alan Kay with a mockup of the Dynabook.
Although the Dynabook was proposed years before the necessary hardware was available, the ideas could be tried out on the Alto, an "interim Dynabook".
Photo by Marcin Wichary, CC BY 2.0.
The Dynabook (seen in mockup above) was to be a low-cost, battery-powered, portable computer with a touchscreen and graphics, able to access information over the network. The system would be highly interactive and programmable in an object-oriented language. As well as the keyboard, voice input could be used. Books could be downloaded and purchased.
Since the necessary hardware was science fiction when the Dynabook was proposed, the Alto was built as an "interim Dynabook" for research.
Butler Lampson's 1972 memo entitled "Why Alto" proposed using the Alto for research in distributed computing, office computing, graphics, and personal computing. He stated,
"If our theories about the utility of cheap,
powerful personal computers are correct, we should be able to demonstrate
them convincingly on Alto."
Xerox used the Alto to research and develop the ideas of personal computing.[4]
Software
The Alto had a large collection of software, largely implemented in the BCPL (predecessor to C), Mesa and Smalltalk languages.
The Bravo text editor (seen below) is considered the first WYSIWYG editor, with formatted text on the screen matching the laser printer output.
Also below is the Draw illustration program which used the mouse and an icon menu to create drawings.
Other significant programs included email, file transfer (FTP) and an integrated circuit editor.
The Alto also ran some of the first networked multiplayer games such as Alto Trek and Maze War.
Bravo was the word processor for the Xerox Alto, providing WYSIWYG text editing.
The Draw program for the Xerox Alto uses the mouse and icons for drawing.
The Alto simulator Salto was used for these images.
Hardware
The Alto was introduced in 1973.
To understand this time in computer hardware, the primitive 4004 microprocessor had been introduced a couple years earlier.
Practical microprocessors such as the 6502 and Z-80 were still a couple years in the future and the Apple II wouldn't be released until 1977.
At the time, minicomputers such as the Data General Nova and PDP-11 built processors out of hundreds of simple but fast TTL
integrated circuits, rather than using slow, unreliable MOS chips.
The Alto was built similarly, and is a minicomputer, not a microcomputer.[5]
The Alto has 13 circuit boards, crammed full of chips.
Each board is a bit smaller than a page of paper, about 7-5/16" by 10", and holds roughly 100 chips (depending on the board).
For the most part, the chips are bipolar TTL chips in the popular 7400 series.
(The MOS memory chips are an exception.)
The image below shows the Alto's card rack and some of the boards.
The Xerox Alto contains 21 slots for circuit boards. Each board is crammed with chips, mostly TTL.
The Alto's CPU consists of three boards.
The Control board is the heart of the processor: it manages the 16 microcode tasks and contains microcode in PROM.
The ALU board performs arithmetic and logic operations, and provides the main register storage.
The Control RAM board provides additional microcode storage in RAM and additional processor registers.
(Note that in a few years, a single chip microprocessor could replace these three boards.)
The photo below shows the ALU board. The 16-bit addition, subtraction and Boolean operations are performed by four of the popular 74181 ALU chip, used in many other processors of the era.
Each 16-bit register requires multiple chips for storage.
The 32x16 register file is historically interesting as it is built from i3101 64-bit bipolar memory chips, Intel's first-ever product.
The ALU board from the Xerox Alto.
The Alto came out at a time when memory was expensive and somewhat unreliable.[6]
In 1970, Intel introduced the first commercially available DRAM memory, the 1103 chip, holding 1 kilobit of storage and making
magnetic core memory obsolete.
The original Alto used 16 boards crammed full of these chips to provide 128 kilobytes of memory.
The Alto we have is a more modern Alto II XM (eXtended Memory) with 512 kilobytes of storage on four boards.
Even so, the limited memory capacity was a difficulty for programmers and users.
The photo below shows one of the memory board, packed with denser 16 kilobit chips.
A 128KB memory card from the Xerox Alto. It uses eighty 4116 memory chips, each with 16 kilobits of storage.
Microcode
The Alto hardware provides a simple micro-instruction set and uses
microcode to implement a full instruction set on top of this,[7]
including some very complex instructions.
The Alto introduced the
BITBLT graphics instruction, which draws an arbitrary bitmap to the display in a variety of ways (e.g. paint over, gray stipple or XOR).
"Blitting" became a standard graphics operation, still be found in Windows.
The Alto takes microcode further than most computers,
implementing many functions in microcode that most computers implement in hardware, making the Alto hardware simpler and more flexible.[8]
Microcode tasks copy every pixel to the display 30 times a second, refresh dynamic memory, read the mouse, handle disk operations, drive Ethernet — operations performed in hardware on most computers.
Much of the Alto microcode is stored in RAM, so languages or even user programs can run custom microcode.
Next steps
The first step to getting the system running will be to make sure the power supplies work and provide the proper voltages.
The Alto uses four complex but highly efficient switching power supplies: +15V, -15V, +12V, and +/-5V.[9]
(Most of the chips use +5V, but the memory chips and some interfaces require unusual voltages.)
The power supplies are mounted in the cabinet behind the card cage, as you can see in the photo below. The +/-5V supply is on the right, and the three other power supplies are on the left.
Looking into the back of the Alto, you can see the four switching power supplies (blue). The card cage is behind them. The disk drive has been removed from the top of the cabinet. On the back of the cabinet, connectors to the display, Ethernet, and other devices are visible.
Restoring this systems is a big effort but fortunately there's a strong team working on it, largely from the IBM 1401 restoration team. The main Alto restorers so far are Marc Verdiell, Luca Severini, Ron, and Carl Claunch. Major technical contributions have been provided by Al Kossow (who has done extensive Alto restoration work in the past and is at the Computer History Museum) and the two Keiths (who have restored Altos at the Living Computer Museum).
For updates on the restoration, follow kenshirriff on Twitter.[10]
Notes and references
[1]
The laser printer was invented at Xerox by Gary Starkweather and networked laser printers were soon in use with the Alto. Y Combinator's Alto is an "Orbit" model, with slots for the four boards that drive the laser printer, laboriously rendering 16 rows of pixels at a time.
[2]
Malcolm Gladwell describes Steve Jobs' visit to Xerox in detail in Creation Myth.
The article claims that Xerox licensed its technology to Apple, but strangely
that license wasn't mentioned in
earlier articles about Xerox's lawsuit against Apple.
The facts here seem murky.
[3]
Amazingly, Alan Kay even predicted ad blockers in his 1972 Dynabook paper:
"One can imagine one of the first programs an owner will write is a filter to eliminate advertising!"
I thought that Alan Kay missed WiFi in the Dynabook design, but as he points out in the comments, wireless networking was a Dynabook feature.
[4]
Xerox called the Alto "a small personal computing system", saying,
"By 'personal computer' we mean a non-shared system containing sufficient
processing power, storage, and input-output capability to satisfy the computational needs of a single user." (See ALTO: A Personal Computer System Hardware Manual.) Xerox's vision of personal computing is described in the retrospective Alto: A personal computer.
[5]
The Alto documentation refers to the microprocessor, but this
term describes the microcode processor.
The Alto does not use a microprocessor in the modern sense.
[6]
Since memory chips of the era were somewhat unreliable (especially in large quantities),
the Alto used parity plus 6 bits of error correcting code to improve reliability.
As well as the four memory boards, the Alto has three memory control boards to decode addresses and implement error correction.
[7]
The Alto's microcode instructions and "real" instructions (called Emulated Instructions) are
described in the Hardware Reference.
The Alto's instruction set is similar to the Data General Nova.
[8]
The Alto has 16 separate microcode tasks, scheduled based on their priority.
The Alto's microcode tasks are described in Appendix D of the Hardware Manual.
The display task illustrates how low-level the microcode tasks are.
In a "normal" computer, the display control hardware fetches pixels from memory.
In the Alto, the display task in microcode copies pixels from memory to the display hardware's 16 word pixel buffer as each line is being written to the display. The point is that the hardware is simplified, but the microcode task is working very hard, copying every pixel to the display 30 times a second.
Similarly the Ethernet hardware is simplified, since the microcode task does much of the work.
[9]
Steve Jobs claimed that the Apple II's use of a switching power supply was a revolutionary idea
ripped off by other computer manufacturers. However, the Alto is just one of many computers that used switching
power supplies before Apple (details).
[10]
Many sources have additional information on the Alto.
Bitsavers has a large collection of Alto documentation.
DigiBarn has photos and more information on the Alto.
The Computer History Museum has a large collection of Alto source code online here.
The Alto simulator Salto is available here if you want to try out the Alto experience.
Wikipedia has a detailed article on the Alto.
Punched card sorters were a key part of data processing from 1890 until the 1970s, used for
accounting, inventory, payroll and many other tasks.
This article looks inside sorters, showing the fascinating electromechanical and vacuum tube circuits used for data processing in the pre-computer era and beyond.
Herman Hollerith invented punch-card data processing for the 1890 US census.[1]
Businesses soon took advantage of punched cards for data processing, using what was called
unit record equipment.
Each punched card held one data record, consisting of multiple data fields.
A card sorter sorted the cards into the desired order. Then a machine called a tabulator read the cards, added up desired fields and printed a report.
For example, a company could have one card for each invoice it needs to pay, as shown below, with fields for the vendor number, date, amount to pay, and so forth.
The card sorter ordered the cards by vendor number.
Then the tabulator generated a report by reading each card and printing a line for each card.
Mechanical counters in the tabulator summed up the amounts, computing the total amount payable.
Many other business tasks such as payroll, inventory and billing used punched cards in a similar manner.
Example of a punched card holding a 'unit record', and a report generated from these cards. From Functional Wiring Principles.
The surprising thing about unit record equipment is that it originally was entirely electro-mechanical, not even using vacuum tubes.
This equipment was built from components such as wire brushes to read the holes in punched cards, electro-mechanical relays to control the circuits, and mechanical wheels to add values.
Even though these systems were technologically primitive, they revolutionized business data processing and paved the way for electronic business computers such as the IBM 1401.
How a sorter works
A card sorter takes punched cards and sorts them into order based on a field, for example employee number, date, or department.
One application is putting records in the desired order when printing out a report.[2]
Another application is grouping record by a field, for instance to generate a report of sales by department:
the cards are first sorted based on the department field, and then a tabulator sums up the sales field, printing the subtotal for each department.
To sort punched cards, they are loaded into the card hopper and fed through the sorter. Cards are read and directed into one of the 13 card pockets: 0 through 9, two "zone" pockets, and a Reject pocket.
This is very different from a typical sort algorithm — cards aren't compared with each other — so you may wonder how this machine sorts its input.
IBM Type 80 Card Sorter.
Card sorting uses a clever technique called
radix sort.
The sorter operates on one digit of the field at a time, so to sort on a 3-digit field, cards are run through the sorter three times.
First, the sorter deposits the cards into ten bins (0-9) based on the lowest digit of the field.
The operator gathers up the cards from the bins in order (0 bin first and 9 bin last) and they are sorted again on the second-lowest digit, again getting stacked in bins 0-9.
The important thing is that the cards in each bin will still be ordered from the first pass: bin 0 will have cards ending in 00 first, and cards ending in 09 last.
The operator gathers up the cards in order again, yielding a stack that is now sorted according to the last two digits.
The cards are run through the sorter a third time, this time sorting on the third-lowest digit.
After the last run through the sorter, the cards are in order, sorted on the entire field.
The radix sort process is fast and simple. You may be familiar with comparison-based sorting algorithms like quicksort that compare and shuffle entries, taking O(n log n) time. Radix sort can be implemented with a simple electric mechanism (along with an operator busily moving stacks of cards around), and takes linear time.[3]
Although the sorter's hopper can hold 3600 cards, it can sort as many cards as desired, as long as the operator keeps loading and unloading them.
The sorting mechanism
You might expect a sorter to have multiple sensors to read the holes from a card and 10 flippers to direct the card into the right bin. But the actual implementation of the early sorters is amazingly simple and clever, using a single sensor and a single electromagnet.
An IBM punched card, showing the encoding of digits and letters.
The photo above shows the layout of a standard IBM punched card, which stores
80 characters in 80 columns. The characters are printed along the top of the card and the corresponding holes are punched below.
For a digit, each column has a single punch in row 0 through 9 to indicate the digit in that column.
(I'll explain the two additional "zone" rows for alphabetic characters later.)
The diagram below shows how the card sorter works.
Cards are fed through the sorter "sideways" starting with the bottom edge (called the "9-edge" because the bottom row is row 9).
A small wire brush (red) detects the presence or absence of a hole; the brush will contact the rows in order from 9 to 0. An intact card blocks the wire brush from contacting the metal roller. But if there is a hole in the card, the brush makes contact with the roller through the hole, completing an electrical circuit.
Card sorting mechanism in the IBM Type 80 and Type 82 card sorter.
A stack of metal guides (called chute blades) is used to direct the card into the appropriate bin.
As a card is fed through the sorter mechanism, it slides under the chute blades as shown in the top illustration.
If the brush (red) makes contact through a hole, it trips an electromagnet (purple) that pulls down a metal armature plate (green), allowing the ends of the chute blades to drop down.
This causes the card to go above the chute blade rather than underneath it.
The key is the chute blades have the same spacing as the rows on the card
so the hole is detected just before the card reaches the corresponding blade.
(If no hole is detected, the card passes under all the chute blades and into the Reject bin.)
For example, in the diagram above the card has slid under chute blades 9 through 5.
The brush makes contact through hole 4, energizing the electromagnet and causing the blades to drop
just before the card reaches blade 4. Thus, the card is directed into chute 4.
The chute blades can be seen in the photo below; they are the metal strips running down the center of the sorter between the feed rollers. Each chute blade ends at the appropriate pocket, causing the card to drop into the right location.
IBM Type 82 Card Sorter. The feed rollers under the glass top send cards through the sorter. The pockets at bottom collect the cards. This is a German model, thus the 'Sorteirmaschine' label.
Alphabetic sorting
Numeric values have one hole in a column and are straightforward to sort, but how about alphabetic characters?
In addition to the ten numeric rows 0-9, punched cards also have two additional "zone" rows (11 and 12).
The diagram below shows the encoding; a letter combines
a digit punch (1-9) with a zone punch (a hole in 0, 11 or 12).
Confusingly, row 0 is used both as a zone and a digit.
The IBM punched card code, from IBM 82, 83, and 84 Sorters Reference Manual.
With this encoding, a sorter can perform an alphabetical sort in two passes.
The first pass sorts on the numeric rows, putting cards into bins 1 through 9.
These bins are gathered up in order and the cards are sorted a second time.
For the second sort, the zone rows (0, 11 and 12) are read and the digit rows are ignored.
The result is A through I sorted in bin 12, J through R in bin 11, and S through Z in bin 0.
For multiple-character fields, the process is repeated for each column.
Control switches on the sorter select a numeric or zone sort.
The photos below show these controls on the Type 80 (top) and 83 (bottom) sorters.
The Type 80 sorter has a round commutator with tabs that are moved in or out to select which rows to use; the red tab selects a zone sort. The Type 83 sorter has pushbuttons to select rows, as well as a switch to
select different types of sorting (Numeric, Zone, or Alpha).
Sorter controls on the Type 80 (top) and Type 83 (bottom) sorters.
A brief history of IBM's horizontal sorters
Type 80 sorter
In 1925,
IBM introduced its first horizontal card sorter, the Type 80.[4]
This sorter became very popular with
10,200 units in use by 1943.
IBM continued to support this card sorter until 1980, a remarkable lifespan of 55 years.
IBM Type 80 punched card sorter.
The Type 80 sorter performed useful data processing with electromechanical technology without the benefits of transistors or even vacuum tubes.
The Type 80 sorter used a relay to latch the electromagnet on for the duration of the card; this is the extent of its "intelligence".[5]
Even though it was electrically simple, the sorter was a piece of precision machinery.
It sorted 450 cards per minute, so the chute blades must pop down and up more than 7 times per second. Any timing error could result in a mis-sorted card or could cause the blade to nick the edge of the card.
Type 82 sorter
IBM's next sorter model was the Type 82, able to sort 650 cards per minute, and renting for 55 dollars per month.
At the faster speed, an electromechanical relay wasn't fast enough to control the magnet, so vacuum tubes were used.
IBM Type 82 punched card sorter.
Type 83 sorter
The next sorter model, the Type 83, was introduced in 1955. It could sort 1000 cards per minute and rented for 110 dollars per month.
This sorter used a much more advanced technique for processing cards: instead of selecting the card chute at the instant a hole was detected, the 83 sorter read all the holes in the column before selecting a card chute.
This allowed the Type 83 sorter to perform tasks that were impossible with the previous sorters, such as rejecting erroneous cards that had multiple holes in one column.
IBM Type 83 card sorter.
Type 84 sorter
IBM's most advanced sorter was the Type 84,
introduced in 1959 and produced until
1978.
This sorter replaced the wire brush with a photoelectric sensor and used solid state technology.
A vacuum feed grabbed cards more effectively.
With these improvements, it could process 2000 cards per minute,
over 30 cards per second flying through the sorter.
IBM Type 84 card sorter. Photo courtesy of Computer History Museum.
Sorters and IBM's industrial design
As you may have noticed from the photos above, IBM's industrial design changed drastically from the early sorters.[6]
The Type 80 sorter is an example of IBM's early hardware, built
of cast iron in a "Queen Anne" style with curved cabriole legs. The mechanisms and motor of the Type 80 sorter are visible.
By the time of the Type 82 sorter, IBM
was using industrial design firms and had
an "understated Art Deco aesthetic". Note the curved, sleek enclosure of the Type 82 sorter, and its shiny horizontal metal trim.
The Type 83 and Type 84 sorters are more boxy, without the decorative trim, moving closer to the dramatic modernist style of IBM's computers of the 1960s.
The technology inside the sorter
This section looks inside the Type 83 sorter and describes how
it was implemented using tube and relay technology.
Unlike earlier sorters, the Type 83 sorter read the entire column before selecting the bin for the card.
This permitted more complex processing, such as detecting erroneous cards with multiple punches.
The sorter used 12 vacuum tubes to store the holes in the column as they were read.
Electromechanical relays implemented the decision logic to select the bin, and then solenoids activated the chute blade for that bin.
Removing the panel from the end of the sorter shows most of the mechanism (below).
At the top is the feed hopper where cards are fed into the sorter.
On the right, a pulley connects the feed mechanism to the motor.
Mechanical cams (behind clear plastic) are also driven by the motor.
Below the power switch and fuses, the 12 vacuum tubes are barely visible.
Two rows of rectangular relays provide the control logic for the sorter.
Behind the relay panel is the power supply for the sorter.
Inside the IBM type 83 card sorter. At top is the card feed. The cams are behind clear plastic.
There is no clock for the sorter; all timing is relative to the position of the driveshaft, with one 360° rotation corresponding to one clock cycle.
Sixteen cams (behind plastic near the top of the sorter) open and close switches at various points in the cycle to provide electrical signals at the right times.
The photo below shows the brush and the chute blade selection solenoids.
On the right, you can see the pointer that indicates the selected column.
The brush itself is below the pointer.
In the middle are the 12 oblong coils that select the bin. These coils push the selected chute blades down (using the levers at the front), allowing the card to pass between the selected blades.
Brush and sort mechanism in the IBM type 83 card sorter.
The card is read by a brush that makes electrical contact through a hole in the card.
The brush is positioned to the proper column by manually turning a knob that rotates the worm screw and moves the brush.
As you can see in the photo below, the small brush contacts the metal contact roll.
Brush mechanism in IBM Type 83 card sorter.
The photo below shows the drive rollers that feed cards through the sorter, dropping them into the appropriate bins, as directed by the chute blades.
The chute blades are barely visible; they are the inch-wide metal strip on the right.
The chute blades are stacked together, with just enough room for a card to pass between them.
Feed rollers and bins for the IBM type 83 card sorter. Cards enter at the far end. The chute blades are the inch-wide strip of metal to the right of the feed rolls.
In order to read a column before selecting a chute, the sorter needed a storage mechanism to remember the 12 hole values. This mechanism is an interesting combination of mechanical switches, vacuum tubes and relays.
Type 2D21 thyratron tubes in the IBM Type 83 card sorter. Each tube stores the presence of one hole.
Each bit of storage used a 2D21thyratron tube.
This interesting tube is about 2 inches tall. Unlike a regular vacuum tube, it contains low-pressure xenon.
If the tube is activated (via its two control grids), the xenon ionizes, causing the tube to remain on until current through it is interrupted. Thus, the tube can be used for storage.
Each tube is in a pull-out module that has the necessary resistors at the bottom.
As each card row passes under the brush, the corresponding thyratron is selected.
Rotating cams attached to the driveshaft mechanically activate switches at the right point in the cycle to select each thyratron.[7]
It seems strange to combine high-speed tubes with mechanically operated switches, but cam-based timing was common in that era.
Once the column has been read into the thyratron tubes, the hole pattern is transferred to relays for "processing".
Relay logic
Unlike the older sorters, the Type 83 sorter reads the entire column before selecting a bin. This lets it, for instance, reject erroneous cards with multiple punches in one column.
How does it detect multiple punches? Instead of using logic gates built from tubes or transistors, it uses a network of relays. This section describes how relay logic works.
IBM relay (permissive make type).
A relay (shown above) contains an electromagnet coil that moves contacts, switching circuits on or off like a toggle switch.
In a typical relay, the circuit connects to the "normally closed" pin when the relay is inactive, and connects to the "normally opened" pin when the relay is active. A relay may have multiple sets of these contacts.
The diagram below shows how a relay appears on IBM schematics. On the left is the electromagnet coil, and on the right is one set of contacts. The diagram shows the inactive state, with the center wire touching the bottom contact. When the relay is energized, the center wire moves and touches the top contact, switching the circuit.[8]
Symbol for a relay: relay number 9 and contact set 2.
The diagram below shows the relay circuit in the sorter that counts the holes and determines if zero, one, or more holes are present.
With no holes (top), current flows along the bottom path.
A single hole (middle) energizes a relay (#7 in this case), transferring current to the middle path.
The next hole (bottom) energizes a second relay (#5 in this case), transferring current to the top path.
Thus, this chain of relays determines the number of holes present, and erroneous cards can be rejected.
Relay network in the IBM Type 83 card sorter. This circuit determines if the card has 0, 1, or more holes.
A more complex relay circuit was the optional faster alphabetic sorting feature available on the Type 83 sorter.
For an additional $15 a month rental fee, customers could sort the most common letters in one pass, saving time while sorting.
This circuit used several large relays, each with a dozen sets of contacts (an unusually large number).
These relays decoded the hole pattern to determine the specific character and then selected the appropriate bin.
The diagram below shows a small part of the circuit.
Detail from relay network for enhanced alphabetic sorting in the IBM Type 83 card sorter.
The photo below shows the wiring on the back of the relay panel.
The wiring in the sorter is all point-to-point wiring, rather than printed circuit boards.
Note that the wires are carefully laced into neat bundles.
Wiring inside the IBM type 83 card sorter. This is the back of the relay panel.
The power supply
When the Type 80 sorter was introduced, standard AC power hadn't fully taken over and parts of the United States used DC or 25 Hertz AC.[9]
Thus, the sorter needed to handle fifteen different line inputs including unusual ones such as 115V DC or 230V 25 Hertz AC.
Internally, the sorter circuits used 115V DC, a rather high voltage for "logic" circuits.
If the line voltage was AC, the power supply used a transformer and selenium rectifiers (an early form of diode build from stacks of selenium disks) to produce DC.
The Type 81 power supply was considerably more complicated since its vacuum tubes required -40V DC. To create this voltage, the power supply used a vacuum tube oscillator, another transformer and vacuum tube diodes.
Power supply for the IBM Type 83 card sorter. Filter capacitors are at top. The power transformer is on the left. Selenium rectifiers (left and right) are built from stacks of selenium disks.
By the time the Type 83 sorter was introduced, AC line power was almost universal, so a transformer could replace the oscillator power supply.
The picture above shows the power supply in a Type 83 sorter, showing the large power transformer (left), capacitors (orange cylinders),
and selenium rectifiers (gray finned objects at lower left and right).
Needless to say, modern switching power supplies are much more compact and efficient than the early power supplies used in the sorters.
Before computers existed, businesses carried out data processing tasks by using punched cards and electromechanical equipment such as the card sorter.
Card sorters remained useful in the computer era and were still used until punched cards finally died out.
Sorters used a variety of interesting technologies from mechanical brushes and cams to relay logic and thyristor tubes.
Even though punched cards are now obsolete, their influence is visible whenever you use 80-column text.[5]
The Computer History Museum in Mountain View demonstrates a working card sorter weekly, so stop by if you're in the area.
Thanks to the IBM 1401 restoration team and the Computer History Museum for access to the sorters.
[1]
Herman Hollerith is one of the key inventors of the data processing industry.
He founded a company that, after various mergers, became IBM in 1924.
Hollerith's 1889 patent 395,782 (Art of Compiling Statistics) describes how to record data on punched cards and then generate
statistics from those cards.
Hollerith also gave his name to the Hollerith constants used for character data in old FORTRAN programs.
[2]
Using a sorter to order cards for a report is
roughly analogous to a
database ORDER BY operation.
Sorting cards so subtotals can be computed is analogous to
a GROUP BY operation.
[3]
Strictly speaking, radix sort on n records takes O(m*n) time if the field is m characters wide.
But since punched cards limit m to 80 columns, m can be considered a constant factor, making radix sort linear.
[4]
The Type 80 card sorter was invented by Eugene Ford in 1925 and received patent
1,684,389 (Card feeding and handling device).
The card sorter has many interesting features so it's a bit surprising that the patent
covers just the "picker" that feeds cards through the sorter one at a time.
The drawing below is from the patent, and can be compared with the photo of the sorter.
IBM card sorter, from patent 1,684,389 (Card feeding and handling device), 1928.
You might wonder how the Type 80 card sorter was introduced in 1925 when
the modern punched card was developed a few years later in 1928.
The first Type 80 sorters worked with 45-column cards and were
slightly modified in 1928 to support 80-column cards.
The changes were minor since the cards remained the same size; the brush mechanism needed to have 80 stops instead of 45.
[7]
A primitive but complex mechanism is used to select one thyratron tube as each row is read.
Although the 12 thyratrons are physically installed in a line, they are electrically wired in a 3x4 grid.
Four mechanical cams select a grid row; one cam is activated at a time.
You'd expect three cams to select a grid column, but there are six.
The problem is a single mechanical cam can't turn the switch on and off fast enough.
The solution is to use two cams in series with staggered operation. The first cam closes the circuit to select the thyratron, while the second cam opens a short time later to de-select the thyratron. By using two cams and two switches, each switch has more time to open and close.
As a card is read, the cams open and close, selecting each thyratron in sequence to hold the value (hole or no hole) for that card position.
After the card column has been read into the thyratrons, the hole pattern is transferred to 12 relays and the thyratrons are reset for the next card.
[9]
The story of why parts of the US used 25 Hertz power instead of the standard 60 Hertz is interesting.
Hydroelectric power was developed at Niagara Falls starting in 1886.
To transmit power to Buffalo, Edison advocated DC, while Westinghouse pushed for polyphase AC.
The plan in 1891 was to use DC for local distribution and (incredibly) compressed air to transmit power 20 miles to Buffalo, NY.
By 1893, the power company decided to use AC, but used 25 Hertz due to the mechanical design of the turbines and various compromises.
In 1919, more than two thirds of power generation in New York was 25 Hertz and it wasn't until as late as 1952 that Buffalo used more 60 Hertz power than 25 Hertz power.
The last 25 Hertz generator at Niagara Falls was shut down in 2006.
See 25-Hz at Niagara Falls, IEEE Power and Energy Magazine, Jan/Feb 2008 for details.
