8085 IO ports

Adrian Graham witchy at binarydinosaurs.co.uk
Tue Jan 17 13:15:57 CST 2017

On 17/01/2017 16:53, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:

> Z80, PDP8, P850, etc) you have special I/O instructions accessing I/O devices.
> The address spaces are totally separate, I/O location 0 has nothing to do with
> memory location 0. On the 8085, an I/O instructon (IN or OUT) will cause
> IO/M to be asserted (other state from when the CPU is accessing memory).

OK. I'd have thought in this case the D8741A would count as a non-memory
mapped I/O device?
> Note that on a machine with I/O instructions (like the 8085) there is
> (a) nothing
> to stop you having memory mapped I/O (that processor can access memory),
> and (b) nothing to stop you having a mix of memory mapped and I/O mapped
> I/O. You might have simple devices mapped as I/O ports, but video memory
> (which is a sort-of I/O device in that storing something there causes it to
> appear on the screen) memory-mapped. As an aside, the TRS-80 model 1
> had almost everything (video, keyboard, printer port, etc) _memory mapped_,
> the only standard I/O mapped device was the cassette unit.

My video RAM is a pair of 2114s which the teletext processor should be
looking at as a page store, I'm still chasing down your idea that it should
be initialised somehow before it starts producing a video sync.

>> They're the big green rectangles visible in this picture -
> Ah.... I can see what appear to be thick-film resistors on them
> (the black rectangles). Are there more conventional components
> on the underside?

Yep, on the left two. I haven't dared to pull them from the board though
even though they're socketed.
>> through exposure to moisture for several years but could they have exploded
>> instead? The damage looks old so I don't think that power up is responsible.
> What is the tape drive? That board has a distinct look of Philips about it.
> What tapes does it use? If I were a gambling man I would guess at Phlips
> minicassettes (not microcassettes). I think I know that drive...

The drive has no maker stamp on it but I think you're right with the
minicassettes - the ejecting lid looks bigger than a microcassete. As it
happens there's an identical one on ebay right now!


Pictures are JUST at the wrong angle to find anything useable on the caps!

> If it is the drive I am thinking of, I have one somewhere, meaning I can look
> up the capacitors.

Cool. They actually might be readable - the detritus that I thought was
exploded cap looks like it might be external muck so I'll clean it up and
see what I can get.
> But I would expect the thing to produce video without it.

Yes, I'd expect video or at least a sync as soon as it's been running for
300usec which is what the datasheet says is all it takes to work out it's
running in 'off hours' mode and to generate its own sync. One thing that CAN
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.
> I don't suppose you have a logic analyser? This is the sort of problem
> that would
> have me using said instrument to see what the processor is executing.

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.


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

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