Post #4385201

2026-08-04 22:36 UTC

As cool as that was, we also learned the keyboard is totally fine, so the issue is elsewhere I then looked more into the minibus backplane, and what a better way to get used to something than making it into a KiCAD symbol and schematic to hopefully make a single extender! https://forgejo.crashtest.dev/jasonalexander-ja/tek-minibus-kicad (I didn't know about the 3.96mm pin pitch standard and I spent hours with board and a vernier and managed to get to 3.97mm... which I am proud of) This did allow me to better understand the architecture of this though, and specifically I thought I should look at the 4.9mHz and 614kHz clocks signals produced by the TC-1 logic board, 4.9mHz was OK (after I stopped checking with a 10mHz Tek 222 and used a more powerful scope), but 614kHz... completely still Turned out to just be a dead U9 7437 quad NAND on TC-1 which droves the line by pulling it down, sadly all I had to hand was a 7400, which is the same chip but rated for less current required for driving a line, so I soldered in a socket and placed it in, and what ya know, 614kHz came riiiight up, probably wont last, but with a replacement on the way and the chip socketed it certainly works for now! As for what effect this had, we now had serial communication in line mode!

Replies (1)

@crashtestdev@woof.tech 2026-08-04 22:44

It was initially rather confusing, pressing a key seemed to send a random character every time, and this was looking at the scope so serial settings wouldn't account for that But serial in did something interesting, with 9600 7M2, sending a CTRL + G rang the bell! It's successfully reading and processing serial input, but what is happening internally with TX?! There was more symptoms, setting it to LOCAL mode and pressing CTRL + G on the built in keyboard it still wouldn't ring it's own bell, I figured it wasn't reading it's own keyboard correctly And more weirdly, the characters it was sending weren't random, each button on the keyboard pressed repeatedly send a series of 7 bytes starting with it's own value and ending on 0x7D and looping back again The wonderful Philpem to the rescue remembered that the keyboard is multiplexed on the TC-2 board as per the functional block diagram, sure enough he was right! Looking at the multiplexer I saw that it was being controlled by a 4 bit counter, probing that counter showed something very interesting, with each key press terminal side, the counter would increment, before resetting, which explained the 7 character sequence! Looking for the source of this clocking, showed it was coming from CBUSY from the serial communication card, this card is oddly an optional extra and it can work without it, we determined that this was a totally normal signal which indicated that the serial card was busy sending data

Open ##4385200