Thursday, August 2, 2018

Readied everything for Vintage Computer Festival West, transporting to the site tomorrow


One of my planned demonstrations was a Hangman program but it won't work in 4K Basic on the Altair 8800 clone, both due to limited room for program statements and use of syntax that didn't exist in 4K Basic. 

TicTacToe and Prime Number Generator both worked fine. I made another copy of the paper tapes for safety and then switched over to the PDP-8 clone. I booted OS/8 and ran a FOCAL program to play Towers of Hanoi. I also listed the directory for ideas for other possible demonstrations. 

I did notice that the ASR 33 letter O and numeral 0 are both filled in with ink when they print. I believe that gunk has built up in the type cylinder for those. I will use Q-Tip swabs and alcohol, plus tools, to try to clean out the letters.

To support my potential modification allowing the SBC 6120 to read paper tape, I began wiring up the inverter board and hooking it to a relay module. Construction was on a PCB built like a breadboard socket, letting me easily hook up the unused inputs to VCC or ground, add a capacitor across the power rails, and wire the input and output of the one gate I will use.

At this point, I realized that my relay modules allow me to wire up the outputs of the relay to either NC or NO points, thus I don't need the complexity of an inverter. I still decided to use it since this will buffer the UART pin and minimize any impact I might have on its operation.

Next steps to make this work are changes to the SBC 6120 and the teletype. A wire has to be tacked onto a pin of the UART on the main board and routed down to my quickie board as the input. The relay lines are connected to the Reader Run Control board wires, to operate the reed relay on that control board.

Finally, I had to wire up a 120VAC switch to manage the shorting of the output of the Reader Run Control board - it must be shorted for most uses, but opened up to allow the board to work for PDP-8 style systems like the SBC 6120. I found an extension core with a remote switch and used that to wire across the Reader Run Control outputs.

After tacking on the wire to the SBC 6120 UART at pin 19, leading to my newly built board, I cobbled together a test setup. I am using a triple power supply since I need 3.3V for the Altairduino to power the MAX3232 level converter, 5V for the board and relay I build for the SBC 6120, and 12V driven through the relay to operate the Reader Run Control reed relay in the ASR 33.

I could hear the 12V close the reed relay in the teletype, but the circuit I built wasn't working. Occasionally it would start buzzing the small relay board rapidly but it did it when no I/O was occuring through the serial port. Something is wrong and I don't have enough time to debug it.

As a result, I will abandon the attempt to read paper tapes with the PDP-8 clone. I have adequate demonstrations on hand without it. I am therefore done with changes.

I updated the scripts and included pictures of key switches and settings. Multiple copies were printed out and brought along. Next I worked on some signage to place on the table near my exhibit. I have six stands to hold a letter sized page, I just needed content for each.

I picked up the model 15 teletype this morning and brought it over to my place for staging and testing. I chose to test the model 15 while it sat in the back of my car to minimize moving the units around; this is both for my back and the sake of the equipment. I ran a power line out and set up the deskside PC, control box, teletype and prepared for testing.

The intended behavior is for the PC based program to provide a short menu on the teletype, then allow me to print news and weather stories from Reuters and the National Weather Service using their RSS feeds. The program BaudotRSS written by John Nagle is what drives the machines.

The program is seemingly working properly, however the keyboard is encoding incorrectly. I need to type the letters N and W to trigger printouts, but get V and M instead. I pulled the keyboard and manually tested connectivity to the various microswitches for the desired letters.

Offline I am getting the right encoding reliably, but the keyboard is erratic in working. I suspect it is tarnish whose cumulative effect provides too much resistance for the Nagle designed interface box to handle properly. I first swapped to my second box, to see if there is any difference using that one.

The second interface box decodes the keyboard identically - sadly that is incorrectly. I could try to burnish, deoxidize and otherwise clean up all the contacts on my keyboard, but fortunately I discovered that running the BaudotRSS program configured for a keyboardless teletype will simply type out the news feed, item by item.

This is the behavior I want, thus I am set for VCF without having to fix whatever is going wrong with the keyboard. I moved the teletype back a bit in the car so that I could shut the rear and mentally checked that item off. Model 15 demonstrations are ready to go.

I expect that I can set up a secondary configuration file with no news feed defined, also keyboardless, which should print out the weather forecast. I will work this out and test it later. 

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