Monday, July 23, 2018

Fixed SBC6120, made duplicate paper tapes, and know cause of linefeed issue on ASR 33 teletype


I hooked up the Altairduino to the teletype and successfully brought up 4K Basic, loaded a program to print prime numbers from paper tape, typed RUN and saw it run correctly. Still no linefeed function but otherwise it works well.

I switched over to the PDP-8 clone (SBC 6120) but didn't have good results. The output on the teletype was garbled and the machine itself was balky. I opened up to see what was wrong and quickly discovered a loose wire to my UART configuration switch.

I do get some oddly garbled initial text from the SBC 6120 itself, but the output from OS/8 itself is good. I did some testing, including booting OS/8, loading PFOCAL and running the focal program HANOI to play Towers of Hanoi. All looks good here.

I did a test to see if the SBC 6120, as a realistic clone of a PDP-8, would suffer from the same problem reading in paper tape. On a real PDP-8, the IO card buffer gets overrun by the incoming characters from paper tape. To resolve this, the PDP-8 has a modification for the ASR 33 which starts and stops the tape reader motor under program control.

Alas, this requires access to the control signals from the PDP-8 IO card, but all that is available on the SBC 6120 is RX and TX lines. Therefore, I see no way to control the teletype paper tape speed to avoid the buffer overrun problem. My demonstrations with the PDP-8 clone won't use paper tape input. 

I chose to punch a second set of tapes for the two longer programs, Hangman and TicTacToe4K, because they don't depend on the linefeed function which is not working yet. I completed these, which gives me two good tapes per demo program. I can always punch more at VCF if that is needed.

To produce the additional copies, I put the ASR 33 in Local mode, stuck the existing tape in the reader and pushed On for the paper tape punch. Once I turned the tape reader lever to Start, it was reading and copying the input tape onto the output tape. 


The problem I faced today was failure of the printer to advance when the LineFeed button or received character is present. I can make it work if I partially depress the feed pawl, thus I will be looking for missing or broken parts. I had to remove the cover of the machine to see into the mechanism and begin the debugging.

I opened up and began inspecting. Nothing appeared missing or damaged, although I did notice some flaking remnants of plastic of some type on a bracket associated with Line Feed. I pulled off the little chunk that was left and looked to see if a bigger piece had fallen into the mechanism and jammed something. Nothing obvious here. The diagram I had describing the workings of the line feed function didn't show the item in question.

Wonders of the internet! Somebody had the exact same issues as me, and their description pointed to a bit of plastic that must be in place for Line Feed to work. is where I found the writeup. This exactly matched my situation.

When I am through with VCF West, I will be visiting RTTY Electronics, Paul Cembura, for parts for the model 15 and also for the correct bit for this machine. In the meantime, I will fabricate some workaround just as he did. I will be guessing about the offset necessary, but hopefully it is not too critical. My first three tries were off, the last unfortunately lead to LF after every printed character.

I then put in an hour poring over the parts list for the model 33, trying to find the plastic part or even the parts near it, all to no avail. Since there are two other model 33 teletypes at Marc's home, there should be a part he can grab and model a replacement on. I do have a week and a half to search the parts list hoping to locate the official part number. 

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