Week ending Nov 24, 2013 - mostly traveling


I spent half a week in Denver at SC13 trade show, meeting with technology providers, after which I swapped clothes and flew to London where I will speak and meet attendees at one of my employer's conferences next week. I had the weekend free in London, giving me a chance to visit the National Museum of Computing and Bletchley Park.

As well, I had an opportunity to meet with a fellow replica designer, Lawrence Wilkinson, who has a 360/30 implemented in FPGA and a typewriter console. I had hoped to spend Friday evening for this, but hotel issues kept me from the room until early evening and I ran out of time and energy. We met Saturday evening, after I was back from the museum visit, enjoyed dinner and talked about designing and building replicas of antique computers.


The National Museum of Computing has an HP punched card reader which is a slight variation of the Documation model 600, but the drive rollers have disintegrated - the rubber has turned semi-liquid due to long term exposure to ozone which is ubiquitous in most areas due to automobile and industrial emissions. The parts for the reader are no longer manufactured or available, but we have been searching for suitable replacements.

I discovered some wheels on ebay that match the external dimension, with a bit of trimming for the width, but need a bit of work to attach to the axle of the Documation mechanism. They have arrived in the post, allowing me to bring part of the supply to TNMOC for their staff to work on and the remainder at my home to allow me a shot at adapting them. The museum has many decks of cards that should be read and preserved, before the card readers are all out of service and before the card stock itself deteriorates.

Potential replacement rubber in between real rollers to be replaced

Roller to be split along circumferal groove, bored out to match hub, then press fit with adhesive to the hub
Current metal axle is easily pressed out, to allow boring of the opening
I was able to record characteristic sounds of the 1130 that is restored and operating at the computer, thanks to the kindness of Peter Vaughan who allowed me in before opening time where I could make the recordings. The background noise of fans running and noise from transformers and other components was the first clip I produced, as I want this to run continuously on my replica from power-on to shutdown.

I didn't record shutdown or power-up sequences - the start is so immediate I can just launch the background noise, while the shutdown could be handled by fading since it is essentially just the fans coasting to a stop. The remaining recordings were a variety of disk activities, especially long, medium and short seeks. The ramkit drive inside an 1130 used a mechanical detent to stop and lock the arm at each cylinder position, which generates a click-like sound as it engages.

The mechanism is arranged with two racks, offset by one cylinder, each with a solenoid and ratchet. By releasing and reactivating the solenoid on one side, the drive accomplishes a two-cylinder seek. Flipping from one side to the other achieves a single cylinder movement. Thus, any seek on the system is composed of a sequence of two-cylinder moves and a final one-cylinder action if the span to be traversed is an odd distance.

A seek all the way to the end of the drive and back to cylinder zero is thus a buzzing, rasping sort of sound from the rapid clicking of the solenoid and pawl up to 400 times in a couple of seconds. Short seeks of 10 cylinders have a brief buzz sound, while individual cylinder to cylinder movements produce a tick sound.

Peter ran the disk utility program DCIP allowing me to record random seeks, initializations and verifies which used single cylinder moves, and some long seeks. I can combine these files to recreate the desired sound as my DEC RK-05 disk drives silently seek between cylinders.

I took the time to carefully image all the logic diagram pages from the museum, as they contained some valuable information I need to implement the SAC feature that drives the 1403 printer, but also serves as additional triangulation on the 1130 design. Since each machine's logic diagrams are individual to fit its exact configuration, it is only by comparing diagrams of many machines that I can generalize the design approach to permit me to more accurately configure the 1130 system I am recreating.

This is a sample of the logic diagrams I photographed at TNMOC
I then walked through the museum, enjoying its collection of great old machines and components. Once again, I enjoyed watching the Harwell Dekatron aka Witch, a large calculator employing vacuum tubes (valves over here) which lit one of ten rotary positions around the face, thus each tube is a decimal digit.

A replica is under construction of the EDSAC, one of the first computers. EDSAC will join the replica of the Colossus, a system that predates Eniac and represents one of the important first steps of computing along with the Zuse system, the Atanasoff-Berry system and Eniac.

Many other fascinating items attracted me, such as an Eliott machine built using a kind of MTL circuitry- magnet transistor logic. In this case, it used magnetic cores as logic gates, forming OR and AND types of circuits. Pulses on the input windings would flip the core to its 'on' magnetic parity, then its state is read by a plus that moves every core to the off state. Any that were already on generate a short pulse on a sense winding, while those staying off create nothing; the gate is reset to read out its prior state, just like the destructive read of core based memories.

Multiple windings in the 'positive' direction would flip the core on if a pulse arrived on any of the windings (A or B or C). If some windings were in the 'negative' direction, a pulse on that winding would either neutralize pulses on positive windows or flip off a core that was already activated (A and not-B).


I began simulating the adapter code to debug the function as much as I could while traveling without access to the 1130 or the reader.I was able to tighten up the timing accuracy of the adapter, producing signals and behaviors for the virtual 2501 reader that should work properly with the IBM designed adapter logic in the 1130.

It was quite tedious to set up the simulation to cause the signals to arrive that would be coming from the 1130 adapter logic, since they have to occur at specific times related to what the simulated 2501 reader is doing. For example, activating the hopper magnets and feed solenoids to perform one card movement cycle requires them to switch on as a specific detector pulse activates at a certain rotational position of the 2501 machinery, then to switch off when another of those detectors (the feed contact breakers FeedCB1/2/3) is reached.

During the read cycle, but not the run-in that loads a card into the preread station, signals have to arrive to cause recording of timing pulses on the emitter wheel, at the time when the trailing edge of the card clears the preread station photocell allowing it to switch on. This prepares timing to emit a pulse right in middle of each of the 80 card columns as the punched card moves through the read station photocells; that emitter pulse is used to latch in the state of a card column.

I first ran the simulator to determine the timing of the contact breakers on their perpetual 100ms cycle. I then applied signals relative to the start of each 100ms cycle that the simulated 2501 is experiencing with its continually running feed wheel. Now that I have the timing of a run-in and a normal read determined, these are copied as a group onto the end of the simulator input to produce that type of 100ms cycle. If no action is commanded by the program or operator, then we have 100ms idle cycles where only the contact breaker pulses arrive.

I tested run-in and normal reads, seeing all the outputs at the right time, the state of various mechanisms change corresponding, such as the pre-read station status based on cards feeding through the reader.


I want to adapt a line printer to behave like a 1403 model 7 printer, making use of the unaltered IBM adapter code and delivering realistic behavior including timing. However, I don't have enough documentation on the 1403 to make that happen yet.

I may be able to extract information from logic diagrams for an integrated print adapter on a 360/20, although the 1403 used with that system includes features like Universal Character Set (swappable print cartridges) which make it materially different from the model 7 adapter would be.

As well, I may be able to extract some information from the logic diagrams for 1401 machines with 1403 printers attached. Combining several sets of diagrams may allow me to triangulate on a suitable model 7 behavior. I have one, the system from TNMOC, and may get access to the diagrams from a private collector's machine across the bay from my home.

Some of this may be moot, since the 1403 is attached using the SAC feature, hooked to a 360 channel, hooked to part of a 2821 control unit, all contained in a large frame cabled to the 1130. Thus, the details of the printer are hidden by the control unit buried in the 2821, for which I have no ALDs, and all that is built into the 1130 is the SAC which hooks to the 360 channel. As long as I can mimic a 360 channel, I will get the IO commands and be able to shuttle data across the interface.

The only reason the internal operational details may still matter is if I want to make this as timing-accurate as I can, then I should be modeling the rotation of the print chain and other actions of the mechanism in order to know when to present 'device end'.


The light panel with 244 LEDs plus wiring to six lamps on keyboard black plate
Six leds on the left side of this black plate, lit by same circuitry as the display panel
In the middle of mounting the light panel inside the display pedestal enclosure
During testing, before mounting of Emergency Pull panel and plexiglass front panel

The reader is furthest away, with interface board on top, the punch is closest.

I found an ET75 on Ebay at a reasonable price, offered as not working/for parts. I picked it up during a trip last week and brought it home. Initially, something is wrong in the physical keyboard or the adjustment, as the mechanism that accepts a character then resets the key is cycling continuously. I will degrease, lubricate and then look into the adjustments to see if this can be brought back to working condition to act as a 1052 console. I have picked up several big lots of spare parts for ET machines which gives me almost anything I might need to get this and the ET50 I already owned both into working shape. Perhaps they would be of interest to some other hobbyist building a working console panel or machine replica.

ET75 mechanism in need of cleaning and adjustment, maybe more.

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