8085 IO ports

Tony Duell ard.p850ug1 at gmail.com
Tue Jan 17 14:59:51 CST 2017

On Tue, Jan 17, 2017 at 7:15 PM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
> 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?

It's very likely to be I/O mapped as it's only a couple of locations.


>>> 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.

Do be careful, the substrate is a brittle ceramic material...

>>> 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!
> http://www.ebay.com/itm/NO-NAME-3922-536-07860-392253607860-CASSETTE-STATION
> -/300633626181

That's the one. It's a Philips mechanism. I have at least one somewhere. I will
try to find it. I don't think I have the schematic for it, but at
least I can look at the
PCB and find component values.

> 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.

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 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.

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



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