Saturday, July 7, 2018

Finishing up ASR 33, preparing for VCF West exhibition August 4-5


I worked on my ASR 33 a bit today. It still sporadically 'sticks', which I suspect is some old lubrication inside the keyboard module that is holding the universal lever from restoring fully. Until it does, pushing a key does not trip the clutch to run a distributor cycle and serialize the character value.

I watched the H lever which connects the keyboard module to the printer unit and it was properly resetting. No effect from prodding of the H lever, if the keys were unresponsive they remained so during prodding.

It works almost well enough. I can usually get 3-5 characters before a freeze, which releases within about 30 seconds in most cases allowing further keypresses. I ran through every character on the keyboard, in both regular and shifted mode, plus some control characters. They all printed cleanly and worked perfectly.

Typed output
I loaded my paper tape punch with some blank stock and used a forcep to hold the tension spring up to put pressure on the tape. I was able to successfully punch a sequence of characters onto the tape. That was ripped off and placed onto my paper tape reader mechanism. When I flipped the lever to START it ran through the tape and it all printed on the paper exactly as it was first entered.

Some punched tape - manually holding tension spring thus over and under tensioned
At this point, I have the broken part in the punch to work on, the sticky operation which I hope will work itself out over time, and still must verify that it works well with 20ma current loops while in LINE mode. All minor tasks with the teletype essentially fully restored already.


I agreed to bring the teletypes and exhibit at VCF West in early August here at the Computer History Museum. My idea is to connect the ASR 33 to either or both of my vintage computer clones - the SBC 6120 and the Altairduino. The SBC 6120 is a PDP8 clone which leverages the Harris 6120 microprocessor which is a PDP 8 implementation. The Altairduino uses an Arduino Due to clone an Altair 8800 system.
PDP8 clone using Harris 6120 processor chip
The problem is the teletype, which requires 110 baud serial port speed. Neither of the clones run that slow. The SBC 6120 can be configured for 300 at its slowest. Similarly, the Arduino Due lowest serial opening speed is 300. I had to investigate ways to modify these to operate at the proper speed, using the software and hardware of these two clones.

The SBC 6120 uses a small four pin TTL chip to produce 4.9152MHz which is divided down to run the serial port. The hardware has four jumpers to set divisors for 38,400, 9600, 1200 or 300 baud. I found a similar oscillator chip that runs at 1.8432 MHz. That reduces the baud rate with the slowest jumper to 111.7 baud. Typically you can be off by +/- 5% in speed and still work well, so my +1.5% is going to work well.

one of these silver cans is the 4.9152MHz baud rate generator oscillator
The Altairduino serial port libraries do not support 110 baud but reading the spec sheet of the Atmel ARM processor chip shows that serial ports 2, 3 and 4 of the chip can be set as slow as 80 baud and as fast as 5,250,000 baud. It will take some code hacking to reconfigure the serial port, which hopefully I can do with minimum impact on the clone software.

I went to the github repository for the Altairduino and starting investigating the configuration code. I saw that it had entries for 110 baud, which implies that this is indeed able to drive the ASR 33 without modifications. The only question is whether the version I built has this functionality or if I have to update to get it.

Then there is the pesky issue that I built my Altairduino with a bluetooth module on the serial port. I don't believe the bluetooth serial board can handle 110 baud, but I can detach it and run straight RS232 out of it.

I did an experiment. Setting the USB Programming port as my primary console, I configured the serial port (bluetooth) to 110 baud 8N2 and attempted to connect to it  via PUTTY from my PC. I don't know whether the fault was in PUTTY, my bluetooth module in my PC or the bluetooth module in the Altairduino.

However, I have every reason to believe that I can remove the bluetooth module and bring out the RS 232 RX and TX lines. The only issue is that these will be 3.3V logic levels.

To connect these machines to the ASR 33, I need to convert voltages to the +/- 12V RS232 levels and then convert again to 20ma current loop. I have a Telebyte 65 converter box for the latter requirement and only need to add in a MAX 3232 based board. The 3232 chip handles 3.3V on the "TTL" side.

CP2102 USB to serial in front, Telebyte 65 RS232 to current loop in back
To test with a PC, I will use my CP2102 based USB to Serial chip, which can be configure to run at 110 baud, hooking that to the MAX 3232, the Telebyte 65 and the Teletype ASR 33. The only board I don't have on hand is the MAX 3232 which I just ordered. It should be here by the end of the weekend.

The other part of the exhibit will be a model 15 teletype, printing out weather reports and news stories much as they did in newsrooms druing the middle of last century. I already have the software and interface hardware for the model 15 which will drive this demo. BaudotRSS provides the softwre feed at 110 baud and John Nagle's TTYloopdriver module drives the teletype. All I need to do is make sure I have a restored model 15 working properly. 

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