Wednesday, January 24, 2018

IBM 1401 reader-print problem fixed; last batch of disc cartridges from Xerox PARC picked up for archiving


Today we met at Computer History Museum to work further on the sporadic failure of the 'German' IBM 1401 system, where its 1402 reader will suffer Reader Stop errors while processing certain sequences of read card-print line commands.

The failures occur while the command to read a card is slightly delayed by the 1403 printer logic copying the print line out of core memory. That 2ms delay of the request is enough to cause the reader to accept the feed request, but the clutch itself does not engage. Error checking circuitry detects this fault and halts the reader.

We suspected that timing of some switches activated by cams was the cause of the problem. Last week we adjusted the primary cam that determines when the feed request from the CPU will attempt to latch the clutch and rotate the card feed mechanism. This is RC6 - continuously running cams are named RC and hooked to the motor so they rotate any time the motor spins.

The cams have tight tolerances - the times listed for the switch to make and break must be achieved withing +0 to -2 degrees, ergo the cam makes at 270-272 and breaks at 340-342 degrees. I had that set to 270 and 340 last week, but we found that another cam in series with RC6 was blocking the operation.

Cam RL10 should make at 270 and break at 340, with the same margins, but this was off by at least 5 degrees. RL named cams turn only with the card feed mechanism - once to move cards one station forward.

The path is hopper to read check to read station to stacker, one feed cycle per move. Usually, a different card is at each stage of the path - card 1 is moved to the stacker while 2 is moved to the read station, 3 is moved to the read check station and 4 is next sitting in the hopper.

To get to the RL cams, quite a bit of the top and side cover of the 1402 must be removed, but once we had access, I tested the cam. We found two problems - the duration and the start point.

The number of degrees from make to break should be 70. If the switch is too far from the cam, the duration is shorter than 70, while a switch that is too close to the cam will produce a longer duration. A setscrew, held in place by a jam-nut, determines how far away from the cam that the switch can move. We loosened screws, changed the setscrew and ended up with precisely 70 degrees of contact but the start point was more than 5 degrees to late.

The start point is controlled by the rotational position of the cam around the shaft that spins all the RL cams. Two setscrews hold the cam in place, or when loosened allow it to be spun forward or backward. We moved the cam until we had it making exactly at 270 and, thanks to the duration we adjusted earlier, it broke at exactly 340.

With the setting corrected, we closed up the reader and ran our test programs again. We had a trusty hand program which executed read-print-read-print-read-print-read-halt and then branched back to the beginning. The last read in the sequence would suffer a Reader Stop most times, because the clutch failed to engage during that cycle. 

This program ran a dozen times with error. We replaced the halt instruction with a branch to produce a continuously running loop reading and printing. No failure in many hundreds of cards. We then ran an "80-80" listing program which looped, reading a card and printing its contents on the 1403, again running as many times as we tried it with no errors.

We can declare this problem fixed. At some point, it would be nice to adjust all the other RC and RL cams, making everything conform to the specs, but right now we have no symptoms and could turn the system over to the demonstration team for regular use.


We brought the group of 25 cartridges we recently archived over to PARC and were given all the packs they could locate. We had 36 numbered cartridges, tied to their inventory of disks, plus 3 packs that were labeled but not numbered nor listed on their inventory. Finally, on our way out with the boxes, we were handed a cartridge that was on display behind the Alto in their lobby.

Starting Friday we will work our way through the discs until we have them all processed. The inventory of PARC disks lists 11 more cartridges that they can't locate now, mostly belonging to Alan Kay. If they do find them, then we would have one more batch to process in the future. 

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