8085 IO ports

Adrian Graham witchy at binarydinosaurs.co.uk
Mon Jan 16 19:37:36 CST 2017

On 15/01/2017 16:59, "allison" <ajp166 at verizon.net> wrote:

>> I've thought of that which is why I'm chasing down details on the Viewdata
>> chip and the D8741A which I assume is being used as a keyboard controller.
>> There are also 3 modules on the phone side which I can't find anything
>> about, marked "NKT NMC1515", NMC1516 and NMC1517.
> 8741A is likely keyboard controller.  FYI its the eprom version of 8041A
> (the a is important). That part is easy to dump the EPROM and analyse as its
only 1K.

Yep, done that fortunately. My MQP programmer can read it and also the PAL
that does the ROM selection so I know they're both OK.
> You can use a 8048 disasembler on that, nearly the same part save for
> the slave IO structure and a few instructions.

Glen Slick has already done that for me, much better results than what I
could get out of the d48 disassembler.

> So its possible to use those pins (4 of them) as inputs without interrupts
> on all or none as you can read their state.  RST7.4 is also special as
> its edge
> triggered (and transition activates it and it sets a latch) so unlike
> the other
> the state of the pin can be a pulse rather than a LEVEL.

OK, that might explain why there's only two entry points for those interrupt
pins in the code.
> So it seems there is a keyboard interrupt and video (scan line) interrupt
> plus the RTC (time keeping and ?).  You also have phone line events in
> there.

Tonight I discovered the D8741A is a controller for the little microcassette
unit that's seriously not well with rust and damaged/rotted/exploded caps :/
> FYI the software structure is familiar and likely straight out of the
> book for the 8085.
> You are preserving cpu status (AC-PSW), BC, DE, HL pairs, then working
> on the interrupt event.


>> Ok, it never gets interrupted then.
> You would also see /INTA (interrupt acknowledge) trigger.

I don't remember seeing that when I was monitoring all the control lines and
I've just noticed on my drawings I've left out INTA, must rectify that.
> Do find a copy (its definitely on line) of the 8085 users manual,
> september 1978

I'll have a look for that at work tomorrow, there's every chance we've got
it in the library.

> It really sounds like the unit suffered a high voltage transient
> (lighting, ESD, power supply
> over voltage).

Yeah, the previous owner did power it up and got smoke but I thought that
was just the RIFA mains filter popping. Currently I'm up to 6 replaced chips
that all had dead inputs and the startup opamp (ICL7611). Fortunately the
non-replaceable ones are OK.

On the tape drive controller board are a pair of very messy 25V caps that I
thought had rotted because of damp - the tape transport itself is probably
beyond saving through rust - but could they have exploded I wonder.
> of TTL across 12 boards to bring it back to life.  The only MOS device
> (had a hole in it)
> was a 8251A USART to the H19 terminal (also toasted).  Z80 was still
> good and still
> in that system working to this day (along with a 8085A subprocessor).

Strewth, that's some troubleshooting effort!

> Sockets on the other hand have caused me no small amount of bedevilment.
> If its not machined pin and old its likely trouble.

I do wonder about the sockets though they're all turned pin. The RAM refresh
and first ROM socket were badly verdigris'd with the battery leaking all
over that part of the board but they test OK with a DMM.
>>> Hummm...  4116 dram, that means you have external refresh logic or they are
>>> going cheap and doing refersh on a interrupt (or maybe) timed loop.
>> There's an MC2342A doing the refresh and that's looking OK now that I've
>> swapped it. The original chip had no working outputs.
> Ok, blown....  that make what I said earlier of a ESD incident likely.
> Check the DRAM too.

They all LOOK ok in that I get active traces at DIN/DOUT and none of them
get hot so they're hopefully OK. I know from working on a few PET 80xx that
a single bad 4116 can stop the machine booting but I don't want to start
desoldering those unless it's obvious one or more have died.

>> If you guys weren't around to put up with my amateurish questions I'd have
>> never started work on it and it would've remained just another unloved bit
>> of kit on a table in a museum like the other 5 that are known to exist.
> We try.

It's all excellent help!
> I enjoy working with 8048(family), 8085 and z80 have built with them for
> decades
> and do both HW and SW.    Its fun to apply modern software techniques to
> old hardware as often they do it well even if not lighting fast.

I still find it difficult to get my brain to acknowledge that despite their
speed I really am looking at things happening on individual clock ticks,
I'll get there :)


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

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