8085 IO ports

[Adrian Graham]

Back to the Executel after a few days relaxing :)

On 17/01/2017 20:59, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:

>> Yep, on the left two. I haven't dared to pull them from the board though
>> even though they're socketed.
> 
> Do be careful, the substrate is a brittle ceramic material...

I don't think I'll touch them, they're obviously part of the phone subsystem
and I doubt I'll ever plug it into a phone line - I have several One Per
Desks for that sort of thing if needs be.

>> be seen on both the built-in TV and my external CUB despite lack of sync is
>> for all intents and purposes random crap which again makes me think
>> something isn't initialising properly.
> 
> Yes. It sounds like the processor is not initialising the video system,
> clearing video RAM, etc.
> 
> Now either the processor is waiting for an interrupt (but from what)? or
> ir's not running the right code. CPU trouble, ROM troublem, RAM
> trouble, address decoder trouble?

I remember becoming aware recently that it USED to look like it was reading
all four ROMs but now just pulses ROM1. I'm still chasing down one
particular clock source because there's a flip-flop LS74 that isn't getting
anything at all, it's fed from an LS21 and an LS139 decoder which in turn is
fed by A4 and A5 on the address bus. Having the whole board drawn out with
all its connections is one thing but I need to put it into a logical
schematic order.

>> I have, a small 16 channel one that's Saleae Logic compatible so I'm slowly
>> learning how to drive that too. I should be able to decode addresses and
>> suchlike using it shouldn't I.
> 
> Can you clock the analyser from an external input rather than sampling
> every 10us or whatever? If so, clock it from the Rd/ signal and grab the
> 16 address lines (8 on the processor pins, 8 on an address latch, most
> likely a 74LS373, which you will have to find!). Now you can see the
> sequence of locations that the CPU is reading. Most (but not all) will be
> instructions. Find one, compare with the listing, see if the sequence makes
> sense.

I can trigger it on the RD signal yes. There's 2 LS373s which both had dead
inputs so I replaced those and made sure the signals were correct at all
outputs. I'll have to lose A15 to latch onto RD (only 16 channels) but I'll
give it a go.

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?