A little while ago I got the offer of some help with this underrated (outside the UK) marvel and with the PDT-11/150 awaiting new drive belts and a new floppy disk, I decided to get it back on the bench.
You may remember that I picked up a PCB for Grant Searle’s Multicomp from Neal, a fellow retro enthusiast and all round “good egg”.
It is Neal who has again come to my aid.
Let’s look at the video.
In a previous post I took out one of the single-bit video RAM chips to see what happens. It showed that the screen data was coming from the RAM (or at least the socket) rather than being invented further down stream by a fault.
We’ve taken that approach a bit further.
Remove all of the video RAM. Now, without those RAMs the screen should fill with 0x7f (actually 0xff of course but it’s 7 bit video). It does.
Next, try grounding pin 12 of each of the sockets for ICs 21-27. Pin 12 is data out. As the pin floats high with no chip present, me grounding it pulls it low and so we should see…
- IC21 0x7e
- IC22 0x7d
- IC23 0x7b
- IC24 0x77
- IC25 0x6f
- IC26 0x5f
- IC27 0x3f
And that’s exactly what I see. This means no shorts on the data bus.
Next the standard ROM is replaced by a special I wrote that just fills the whole screen with 0x00 and stops by looping back on itself.
Repeating the above test but but substituting one RAM chip for tapping the wire I should get exactly the same results.
Nope. IC25 and IC26 didn’t work as expected but the others did.
A slight change of tack.
But only slight. More an investigation based on matters arising.
I need a new ROM. Neal has a little piece of code that should generate this…
Let’s go round again.
This time, we’re going to try Neal’s test code but with all of the video RAMs in but a number of other chips removed.
IC28 (video RAM read buffer) IC47 (user RAM read buffer), IC35 (PIO), IC40 (kbd rd) and IC29 (UART).
Power on and we get…
Put a few things back – UART and PIO.
I put back the UART and the PIO as I figured these two were unlikely to be the cause. Sure enough the picture came up just as before.
Now IC40, kbd read. Still fine.
Replacing the buffers.
I put back IC28 the video RAM read buffer and it’s still fine. The reason that the picture is different to the previous is that the software reads back the fist byte of video RAM as a seed value. Without IC28, the video RAM read buffer, it can’t see what’s going on and so always starts with the same byte.
With the RAM buffer in place, each time it’s reset, it increments the first value so that successive presses of the reset button cause the pattern to creep. I don’t know if it’s significant the the first byte read without the buffer appears to be 0x08. I was expecting 0xff.
The test is working. Now what?
Back to my zero fill ROM.
This doesn’t make sense. Time to check the code again.
It seems I’ve mucked it up a bit. Here’s my code.
org 0x0000 start: ld hl, 0x0800 ; Start of the screen ld e, l ld d, h inc de ld bc, 0x0400 ; Size of screen memory ld a, 0x00 ldir loop: jp loop end
See how I carefully load the accumulator with 0x00 and then forget to put it into (HL)? Doh!!
The intention was to clear the first byte of screen RAM and then copy that throughout the memory. Without the first byte being set, we’re just filling with whatever is in that first byte. It could be anything.
It doesn’t explain why the screen is filling completely now.
Keep testing – single char fill.
Neal’s video test.
So back to Neal’s video test ROM again, still with all of the chips in.
It’s completely reliable.