Sunday, July 22, 2018

BaudotRSS program fixed and ready to drive model 15; paper tapes punched to drive ASR 33 demos


I wired up a USB link to true RS232 to allow me to directly drive the ASR 33 from the PC, with the intent of producing punched tapes for the upcoming demonstrations at VCF. With this working properly, I attached to the teletype and ran a terminal program which read a text file and sent each line to the teletype for punching.

First to run was a short basic program Prime to list prime numbers. It punched well and seemed to read back just fine. The extremely old paper tape I have on the machine is challenging to force into the punch. I spent some time fighting with the tape and punch until I was ready to punch out the other demo tapes.

Next up was Hangman, a relatively long program. I spent a miserable 20 minutes with the punch repeatedly stopping halfway through the program when the shoddy quality tape that I have on the machine would jam. Each time, I have to rethread the tape and start over. I took a mental health break.

I then remembered that I had bought some paper tape rolls many years ago when I was building a paper tape punch for my IBM 1130 replica. I found the box and grabbed one of the rolls. I threaded it into the ASR 33 and was back in business. I punched out Hangman, TicTacToe4K and another copy of the Prime program.

I did have to hold the spool of blank tape and feed it out in a loop, since there seems to be too much drag when the roll is sitting in the teletype feed slot. I will look into this later, perhaps it just requires some cleaning and waxing of the surfaces to reduce friction.

Hangman, TicTacToe4K and Prime programs on paper tape
The old paper in the teletype is as bad as the paper tape stock - frequently jamming up and blocking the line feed from advancing the paper. I did buy some new paper rolls, but also plan to unroll the old paper. It is two copy paper, although whatever chemical was intended to act as carbon paper has long since evaporated.

I unspooled the entire roll onto the floor and then rolled up each of the two copies separately to produce two 'new' rolls. If these feed well enough as single copies, they are preferable to use at the show. They are the standard teletype light yellow color and the top copy has Western Union Telex printed along the left edge for extra authenticity.

Unspooled two ply paper roll
After a wonderful half hour spent separating and rolling up the outer ply, I had one roll of WU Telex paper ready to install in the machine. I got it lined up and discovered that somehow my line feed (LF) had stopped working. The printer won't advance the paper when an LF arrives!

Now I have something broken to repair again. Very disheartening. I have family obligations which will keep me from doing any work from the 24th to the 31st of this month. With VCF West loading on the afternoon of August 3rd, I only have three full and one partial day left to deal with this.

Bummed out and having a huge pile of paper on the den floor, I had to roll up the second play to make into a usable paper roll. That took about 30 minutes and took my mind off the treachery of the ASR 33 printer.

Second ply in a heap on the floor
The problem with the line feed seems to be something up on the left side of the platen. I suspect that a spring was dislodged by the crappy two-ply paper which was jamming up earlier, or a part broke. The feed pawl (see below) doesn't move when a LF is commanded, but if I push the pawl down a bit there is a zone where it will reliably jump and pull the platen one line up.

Line Feed mechanism
The feed pawl seems to snap back up, so I can rule out loss of its spring. I will be looking at the line feed linkage in more detail, after I remove the covers from the unit. This will have to wait until tomorrow. 

I had some problems with conflicting Python versions on my laptop, which affected the BaudotRSS program which will drive my model 15 teletype. I did a clean install of Python 2.7 on a desktop machine I use for amateur radio and HP 1000 disk emulation, installing all the packages needed by the program.

Everything appears to work perfectly, based on log output and the blinking lights on my teletype interface box. Before VCF West I will have to bring the entire desktop PC over to the model 15, or bring the model 15 over here, and run a final test.

I took some acrylic square rods and glued them to the SBC 6120 panel to fit it to the wooden box I recently bought. This provides a nice case for use during VCF West.

PDP-8 clone


  1. Hi, I do have a Teletype ASR33 as well and the line feed is a problem for me aswell. The LF is always activated when a character is printed, so each char printed ends up in a LF. Did you find out what was the problem in your situation?

    1. Some ASR 33 models have a plastic part that sits on a lever involved in line feed. The plastic gets brittle and falls apart, leading to the symptoms we experienced. You will need to make some part, perhaps of rubber, to replace that plastic.