8085 IO ports

Adrian Graham witchy at binarydinosaurs.co.uk
Mon Jan 16 19:15:22 CST 2017

On 15/01/2017 14:38, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:

> But do you know it''s not doing I/O. OK IO/M is never going into the
> right state for
> I/O, but what that _really_ means is that the 8085 is never executing
> any IN or OUT
> type instructions. But of course memory mapped I/O is possible
> (storing or loading
> at particular locations that happen to be I/O devices) on any processor that
> can
> access memory (including the 8085). I've seen small 8085 and Z80 control
> systems
> with only memory-mapped I/O.

I pondered that too but the reference says IN and OUT are used for
non-memory mapped I/O and there's a few of those instructions in the code.
Whether they're being executed at this point in time is moot.

>> Apologies for all the daft sounding questions (this goes to Dwight, Tony and
>> Chuck too) but historically I'm a high level programmer so getting right
> I don't think I've seen any daft questions yet!

Phew :)
>> 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.
> Why do you assume it's a keyboard controller? It might be, but it could
> be other things; THe 8741 is the EPROM version of the 8041. It's a
> 8048-like microcontroller with a couple of latches on-chip so it can be
> used as a peripheral from another processor (here the 8085). You need
> to trace the I/O lines from the 8741 to see if they go to the keyboard.

I've now traced all of them and its associated pair of supporting chips
(LS04 and an MM74C906) and it's a tape controller, it's only using port 2
and all the lines go to the tape drive header.

>> There are also 3 modules on the phone side which I can't find anything
>> about, marked "NKT NMC1515", NMC1516 and NMC1517.
> Are these potted blocks, or can you see the components on them?

They're the big green rectangles visible in this picture -
The D8741A is above them and the SAA5070 LUCY chip is to the right.

> Could this be part of the serial data transfer? There will be incoming data
> at 1200 baud. There should be some kind of demodulator (maybe one of the
> modules) and a serial-to-parallel converter You've not mentioned a serial chip
> (is there one), if not then I would expect it to be simulated in software.
> Maybe on the 8085, maybe on the 8741.

LUCY does that, it's also where the keyboard connector's lines split off so
the whole data bus goes up to the keyboard module too. I now need to check
that for power and dead chips since it's obviously not the passive device I
thought it was previously. I've tried running the machine with the keyboard
connected but there's no indicators on there to show any signs of life.

>> RST6.5 is from the Teletext chip, wired to the video status outputs.
>> RST5.5 is from the D8741A, again from a pair of status outputs (IBF/OBF)
> The last basically says that either the 8741 has data to send to the 8085
> (OBF = Output Buffer Full) or is ready for data from the 8085 (IBF = Input
> Buffer Full)

Yep, read/write to the tape device which I can't connect up until I've
replaced the rotted caps.

Now, having just typed that it's making me think of what Allison said about
lightning or ESD, I know the previous owner of this machine powered it up
before putting it on eb*y and 'the smoke came out' which I thought initially
was just the RIFA mains filter popping (it had), but look at this picture:


These are on the tape drive controller board and I thought they'd rotted
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.


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

More information about the cctalk mailing list