8085 IO ports

[Adrian Graham]

On 21/01/2017 17:30, "dwight" <dkelvey at hotmail.com> wrote:

> It is possible that it is the phone side of the system that is
> hanging it up ( no pun intended ).
> A while back, I purchased about 12 broken phone modems for
> about $2. I found out that all the boards had suffered lightning
> damage. Various parts on each board were blown. There was
> no consistency. I believe I recovered about 5 boards by combining
> the good parts.
> The reason I mention this is that it seems your board has had
> more than one part fail. It is possible that it was lightning
> damaged.
>

I agree, it's also been wet and I don't know if that was pre-failure or just
an effect of long term storage. There are also some (I assume) voice
components like a -24V regulator (MC79L24A) that aren't getting power, I've
just looked at the transformer that feeds it and that doesn't appear to be
working.

I'll post a separate pic of that.
 
> If so, it could be that one of the chips has sent an interrupt to
> the processor but not hand shaking when the processor is
> asking for data from the modem.
> Just a though.

The modem in this case is an SAA5070 "LUCY" chip which I guess I can't
easily test or even source a known working spare if it turns out to be
faulty :/

Cheers


> ________________________________
> From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Adrian Graham
> <witchy at binarydinosaurs.co.uk>
> Sent: Saturday, January 21, 2017 8:27:06 AM
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: 8085 IO ports
> 
> 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?
> 
> 

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