Repairing the card reader for a 1960s mainframe: cams, relays and a take hold of

I honest no longer too prolonged ago helped restore the card reader for the Pc Historical previous Museum’s vintage IBM 1401 mainframe.
In the technique, I learned loads about the venerable nevertheless attention-grabbing electromechanical programs frail in the card reader.
Most of the card reader is mechanical, with belts, gears, and clutches controlling the movement of cards throughout the mechanism.
The reader has a tiny quantity of good judgment, nevertheless in its save of transistorized circuits, the good judgment is implemented with electromechanical relays.
1
Timing indicators are generated by spinning electromechanical cams that generate pulses on the upright rotation angles.
This post explains how these varied pieces work together, and how a refined timing self-discipline brought about the card reader to fail.

The IBM 1402 card reader/punch. The 1401 computer is in the background (left) and a tape drive is on the agreeable.

The IBM 1401 become a accepted commercial computer of the early 1960s, frail for applications resembling payroll or stock.
Info records had been saved on 80-column punch cards, which were be taught into the computer by the card reader and
printed on the high-hasten line printer.
The IBM 1401’s card reader (above) might per chance likely be taught 800 cards per minute—over Thirteen cards per 2d—which is a significant hasten for a mechanical instrument.
Cards whizzed throughout the card reader, 80 metal brushes be taught the holes in every column, after which the cards had been stacked in the hoppers throughout the reader.Three
If something went sinful, from a misread card to a card jam, the card reader would slam to a discontinue (confidently sooner than twisted cards started piling up) and an error mild would illuminate on the card reader’s preserve a watch on panel. At this point, the operator would repair the self-discipline and processing might per chance likely proceed.2

80-column punch cards. Every column of the card holds a character, represented by the holes punched in the column. THE text on the highest of the card reveals what is in every column. Gloomy paper in the abet of the essential card makes the holes extra visible.

The describe below reveals the IBM card reader/punch with the front doorways opened up.
The agreeable half is the reader and the left half is the punch. (The punch records records on cards; I will ignore it listed here.)
The blue panel has the operator controls and indicators.
Cards enter throughout the feeder in the better agreeable, commute agreeable-to-left throughout the brushes (in the abet of the blue panel) and tumble into the stacker slots (center).
To the agreeable of the stacker is the service panel with the dynamic timer (circle with knob); this might per chance additionally additionally be relevant later.
The relays, resistors, capacitors and diodes that homicide up the reader’s circuitry are below the service panel.
Converting the outlet sample to characters become performed by the 1401 computer, no longer the reader, so this circuitry is rather frequent.Three
The rubbish can on the lower left collects « chips », the bits punched out of cards by the punch, and triggers a warning mild when it is tubby.

With the front doorways of the card reader/punch removed, the circuitry is visible. The left half is the punch and the agreeable half is the reader. The *Reader Terminate* mild is illuminated on the panel, indicating a malfunction.

A few months ago a scenario with the card reader saved displaying up below certain circumstances:
the card reader would discontinue with a « Reader Terminate » error, indicating a scenario with card feeding and commute.2
Given the age of the card reader on the Pc Historical previous Museum, or no longer it is no longer exquisite that Reader Terminate considerations come up.
Then again, in this case, cards had been feeding magnificent and there become nothing visibly sinful.
In the slay the 1401 repairs team obvious that
the self-discipline happened if you attain a pair of cycles of reading a card, printing a line on the printer, reading a card, printing a line, and so on. The first three cards be taught magnificent, nevertheless the fourth card be taught continuously failed with a Reader Terminate.
But all the best possible device through same earlier processing, a full bunch of cards is susceptible to be be taught with out a scenario.
What become varied about continuously reading and printing?

The card reader can be taught 800 cards per minute, nevertheless the road printer can most attention-grabbing print 600 lines per minute.
Thus, if you continuously be taught and print, the printer will in the end tumble in the abet of.
The slay end result is that the card reader will in most cases be blocked whereas the printer finishes a line.
In specific, the system can be taught and print three cards in a row, nevertheless there might be a lengthen sooner than reading the fourth card.4
Right here’s same earlier behavior for the system, nevertheless for some motive, this lengthen become triggering the Reader Terminate error.

The quiz at this point of the investigation become why the delayed be taught brought on a Reader Terminate.
The good judgment scheme below reveals the many varied issues can trigger a Reader Terminate.5
(This good judgment become implemented in the reader by relay circuits of Byzantine complexity.)
In overall a card jam or card feed self-discipline is responsible for a discontinuance, nevertheless there weren’t any card movement self-discipline.
And the card levers (microswitches) that detect card movement had been operating because it must be.
In the slay, the team found that laying aside relay R-10 (Learn Snatch Impulse shut to the bottom of the scheme) stopped the Reader Take a look at from occurring.
Removing this relay averted the take hold of location from being checkedeight so it averted the error from being reported nevertheless didn’t repair the underlying considerations. It did, then again, inform that the self-discipline become with the take hold of.

Diagram displaying the good judgment stipulations that yield a Reader Terminate error in the card reader. From the handbook.

At this point, I will point to how the take hold of works in the card reader.
The card reader is powered by a 1/4 horsepower electric motor.
This motor powers a total lot of shafts, belts, rollers and cams within the card reader.
Then again, the reader needs a mechanism to speedy birth and discontinuance card reading below computer preserve a watch on.
Right here’s performed by a take hold of that powers the card feed rollers. With the take hold of engaged, cards would per chance be fed throughout the card reader. With the take hold of disengaged, cards will no longer move (even supposing varied parts of the reader will preserve running).
The GIF below reveals the take hold of in action: it is disengaged for one cycle and engaged for one cycle.
Even supposing the silver pulley (driven by the motor) turns continuously, the black gear is started and stopped by the take hold of, which is staunch in the abet of the black gear.

The take hold of in the 1402 card reader. The take hold of is disengaged for one cycle and engaged for one cycle.

The scheme below reveals the critical parts of the take hold of. The ratchet (inexperienced) continuously rotates, powered by the motor.
When the coil (orange) pulls the latch (red), the dog (blue) will tumble into the ratchet (inexperienced), collaborating the take hold of.
This might per chance additionally trigger the outer wheel to rotate in sync with the ratchet, feeding a card.
After one cycle, the take hold of might per chance additionally additionally be disengaged by the latch, or can remain clutched for yet every other cycle.
Above, you’d additionally think the latch move staunch sooner than the take hold of engages. The dogs and ratchet are hidden in the abet of the black gear.
For extra tiny print, think the footnotes.67

The card reader’s take hold of mechanism. From the 1402 handbook.

The take hold of requires cautious timing. If the coil pulls the latch too insensible, the dog might per chance additionally no longer preserve with the ratchet till the following cycle and no card would per chance be fed.
And if the latch is launched too insensible, the take hold of might per chance additionally no longer disengage till the following cycle, feeding a further card.
Either technique, this would trigger a severe self-discipline if you’re, as an illustration, printing payroll tests.
To take this, the card reader entails a take hold of test circuit that ensures that a take hold of happens most attention-grabbing when requested.eight
This circuit is what brought on the Reader Terminate scenario.

Timing in the card reader is controlled by the angular residence of the critical drive shaft; there might be no longer a clock mark.
A 360° rotation of the drive shaft corresponds to 1 card be taught cycle.9
The mechanical and electrical parts needs to be exactly adjusted so they set off on the factual rotational angles.
Shall we reveal, a card’s first row of holes reaches the brushes at eight° and card lever #1 is brought on at 227°.
And the take hold of is engaged or disengaged at exactly 315°.
Electrical indicators are generated by rotating cams that birth and shut microswitches on the good diploma angles.
Shall we reveal the be taught take hold of is controlled by cam RC6 that closes at 272° and opens at 342°.
The describe below reveals a cam on a drive shaft, with the microswitch above it; when the cam rotates to a high point, the swap closes.

One amongst the cams and microswitches in the card reader.

To think timings, the card reader has an spell binding repairs characteristic called the Dynamic Timer, which is form of a mechanical oscilloscope that visually reveals the attitude timing of indicators.
The dynamic timer features a neon bulb that spins in the abet of a plastic disk labeled with the angles from Zero° to 359°.
If a mark is chosen with a preserve a watch on panel swap, the bulb lights up when the mark is intriguing.1
For the reason that neon bulb is spun by the critical driveshaft, its residence is synchronized to the rotation attitude of the card reader’s mechanisms.
Thus, the neon bulb traces out sectors of a circle, indicating when the mark is intriguing.
Shall we reveal, the image below reveals the be taught brush mark, which is activated as every card row passes below the brushes.
Since there are 12 rows on a punch card, the mark is activated 12 cases in a card cycle.
By reading the attitude labels on the determined plastic dial, the mark timings might per chance additionally additionally be checked for correctness.
As well, the knob throughout the dynamic timer might per chance additionally additionally be turned around manually to maneuver the card reader’s cycle to any desired attitude.

The dynamic timer of the 1402 card reader. The 12 indicators correspond to the 12 rows on the card, indicating when the row passes throughout the brushes.

We measured the timing of be taught take hold of preserve a watch on cam RC6 and located it become off by about 5 degrees.
The handbook called the timing of this cam « serious »
asserting it wished to be within 2° earlier or Zero° later from the factual attitude.10
For the reason that cam become successfully out of doorways the allowed vary, we tried to adjust it.
The first self-discipline become that the cam become frozen to the shaft; it sounds as if after decades of storage in a a moist storage it had rusted to the shaft.
Carl in the end hit the cam laborious adequate (yet in moderation!) with a hammer to loosen it from the shaft so it is far susceptible to be adjusted.

The describe below reveals how we manually adjusted cam timings. The dynamic timing wheel (left) might per chance additionally additionally be become by hand till the cam closes, after which the attitude might per chance additionally additionally be be taught from the dial.
The adjustment job is rather dull, requiring trial and error.
First, the length that contact is closed is corrected by though-provoking the contact nearer or farther from the cam.
After every adjustment, the length needs to be manually checked by slowly rotating the dynamic timing wheel 360°.
Once the length is factual, the following step is to rotate the cam a number of degrees at a time till it closes and opens on the agreeable attitude.
All every other time, the timing needs to be in moderation checked after every adjustment till trial-and-error yields the agreeable cam surroundings.

By manually rotating the dynamic timer wheel, the cam timing (center) might per chance additionally additionally be checked. The mild (better agreeable) reveals when the cam has closed. This describe reveals cam RL10 being adjusted.

Once cam RC6 had the upright timing, we tested the card reader. Surprisingly, it failed staunch as sooner than.
We checked the timing of RC6 with the dynamic timer, and surprisingly, the timer mild confirmed that it closed 5° too insensible, even even supposing we had measured it as closing because it must be.
After puzzling over the schematics, I discovered that cam RC6 become powered through cam RL10, so a depraved timing on cam RL10 would trigger the dynamic timer to suppose the sinful time for RC6.

We continued the following week and measurement of RL10 confirmed it become closing 5° too insensible.
So we repeated the cam adjustment job with RL10.
Carl loosened RL10’s setscrews, whacked it with a hammer, and become bowled over when it spun around on the shaft—it sounds as if it hadn’t rusted onto the shaft the technique RC6 had.
For the reason that timing of RL10 become now totally off, it took some extra effort to search out the agreeable residence for it.
One complication is that RL cams most attention-grabbing rotate whereas the take hold of is tripped,
so we needed to manually outing the take hold of for every test.
(This video reveals how the take hold of might per chance additionally additionally be tripped manually.)
After a pair hours of adjustment, the cam had the agreeable length after which the agreeable birth attitude.

We powered up the card reader and this time it ran the overall assessments efficiently! We had found the elusive self-discipline.
It sounds as if cam RL10 become birth a dinky bit too worthy, so the mark to be taught a card after a cease in reading wasn’t rather in a location to preserve the take hold of.
Then the take hold of test circuitry would detect that the take hold of had failed to preserve, triggering the Learn Terminate.

Conclusion

Tracking down and fixing the mysterious Reader Terminate errors on the card reader become sophisticated and extra time-moving than most considerations.
Then again it equipped an spell binding search for abet in time on the venerable technology frail within the card reader: relay good judgment, spinning cams and a mechanical take hold of.
Debugging the card reader is a weird and wonderful expertise from tracking down machine considerations or even same earlier hardware considerations.
Correct working out the schematics with their venerable symbols on the overall is a scenario.
Confidently what we learned in this restore will succor us the following time the card reader malfunctions.

Many participants of the 1401 restoration team had been fascinated by fixing this self-discipline, at the side of Carl, Frank, Alexy, Ron, George, Glenn, Jim and Grant.
The Pc Historical previous Museum in Mountain Score out about runs demonstrations of the IBM 1401 on Wednesdays and Saturdays so test it out if you’re in the role; the demo agenda is here.

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Notes and references