February 23, 2022

Callan Multibus 68k CPU board - Trouble with the multibus

This is by and large, a record of various ways I have been chasing my own tail. It is probably not of interest to anyone else, but it helps me document things so I don't fall down the same open manhole over and over.

Serial output from the Callan

Should the Callan CPU be sending anything out the serial port?
Will it run properly without other multibus cards?
What baud rate will it talk at?

I have the growing feeling that the boot roms never talk to the serial port. This is based in part by experiment, and even more so by my study of the disassembled roms.

Looking at the boot rom code, I was convinced that the uarts would talk at 19200, but they don't, they talk at 9600, just like the manual says.

Which of the two connectors on the serial ribbon? There is a drawing in the manual that shows channel A with a male 25 pin connector and that is on the cable near the card edge. This matches what I see on my cable. It says that it is configured "DCE" (port B is female and configured "DTE").

It is important to note that the Callan board seems to run properly in that the HALT led is not illuminated. It lights up when you hold the reset button, then goes out and stays out when you release it.

Finally - some serial output

I got the idea of removing the "extension roms" and seeing what happens. I see output at 9600 baud, as follows! 
PM68k Monitor, Version 1.2 - 40000 bytes of memory
>
I thought that I didn't have this ROM monitor (which is described in the manual). But I do! It was just not being run when the extension ROM was in place.

I do need to use a null modem gadget that swaps pins 2/3 and such. I am using a USB to serial gadget on my linux machine along with picocom.

A curious note. I am powering the board using only 5 volts and I am getting uart traffic. I would have expected that to require 12 volt supplies to get the usual +-12 volt levels for RS232. I don't see the usual 1488 and 1489 chips on the board, so they must be getting by with something non-standard. I do see AM26LS32 and AM26LS29 chips near the serial port ribbon.

Trouble with Heurikon boards

I have two Heurikon HK68/ME multibus CPU boards with mc68010 or mc68000 chips. I decided to use these to learn how to set up and use the Promice with the dual ROM arrangement I have never used before. This may have been a bad decision that wasted a couple days of my time.

According to some notes I have when I worked with these boards before, one of them just never has worked. One did work and I developed some simple code that would blink one of the LED's. So the first thing I did was to plug this board into my multibus cage and there was no blinking. There is a cluster of 4 LED's that light up (as they always do on board reset). My code should then turn them all off and then make one of them (LED1) blink. And in the midst of plugging and unplugging the board it actually did begin to work properly. That was 2 days ago and I have never seen it work since. I think there is some kind of intermittent connection on that board. It worked reliably back when I developed the blink demo (in 2013?).

I misinterpreted this as some kind of problem with my multibus card cage. It seemed to work in some slots and not in others, but I now think it is the board itself.

Feedback? Questions? Drop me a line!

Tom's Computer Info / [email protected]