Thursday, July 12, 2018

Testing out Altairduino and SBC 6120 with the ASR 33 teletype


I set up my test rig outside, with the ASR 33, the Altairduino, the SBC6120, and various tools like a scope and power supply. I worked my way out from the Altairduino, identifying the output stream when I attempt to switch over to the serial line as the primary interface. Seeing which line produces proper 110 baud ASCII, I then connected through the various devices and cables until I had the Telebyte driving the teletype.

When I applied the switchover on the Altairduino, the teletype printed out the message requesting confirmation of the new settings. However, it did not recognize my input keystrokes. I would get 30 seconds of prompts and then the Altairduino switches back to its USB link for communications.

I next wired up my USB to serial device (based on CP6102 chip), again identifying which line was modulated as I typed input on PUTTY on the PC. Wiring all that through to the Telebyte, I was ready for the next test. This part worked flawlessly, with every character entered on the PC keyboard printing properly on the ASR 33 and each keystroke on the ASR 33 displayed properly on the PUTTY screen. 
Signals going through MAX3232 converter
The above test lets me know that I have a fully functional ASR 33 and any remaining difficulties are going to be with my two clone computers and the wiring to the Telebyte. Seeing perfect output from the Altairduino is an excellent sign, so the only issue to identify is why I was not seeing the keystrokes coming back. I can diagnose this with a PC to Altairduino link, both sides at 110 baud, until I find the problem. 

My final test was the SBC 6120, which has an RS 232 (TTL) interface that I believe I had set to 110 baud using the new slower TTL oscillator chip. Once again, I identified the modulated pin coming out of the SBC 6120 and wired it through to the Telebyte and the ASR 33. 

The results were not as positive as I had hoped. The teletype went into chattering mode, which happens when it receives start bits with ascii NULL following. When I pushed the keys on the teletype, it interrupted the chattering and tried to echo something back to the printer. 

I put the meter on the cable from the SBC 6120 and realized it was already RS 232 voltage levels and thus inverted. That explains the results I experienced. I can simply wire this replica right to the Telebyte input and it should work perfectly. 

I tested this by wiring it up quickly and booted OS/8, then typed some commands. The input seems to be recognized perfectly but the output characters are pretty mangled. There are sections where intelligible portions of a file name show up and then we are back to junk. 

My suspicion is that the SBC 6120 is properly speaking at 110 baud but not set up properly for 7 bits, MARK parity and 2 stop bits. Without enough stop bits, the data streams in faster than the ASR 33 is ready to interpret characters. I will read up a bit to see what I need to do before this will work correctly. 

To diagnose the Altairduino, I wired my USB to serial connection through to the Altairduino serial port, brought up PUTTY on the PC and tried to switch over to the serial line to see what was up. I did manage to switch over and communicate on the serial line, running at 110 baud. This works fine talking TTL level RS 232. 

I probably fried my MAX3232 board since I applied +/- 12V signals to the TTL/LVCMOS side. I tried a quick test and indeed the module is dead as a doornail. I ordered another one and expect it tomorrow night At the same time, I ordered some DB9 and DB25 breakout modules to allow me to wire up everything for VCF. 

I will make up the final connections this weekend and then do one a live test with the ASR 33. Before I bring it all to VCF West, I need to script out some demonstrations and write it up, so that my booth assistants can run things whenever I am away from the area. 

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