On Tue, Jan 17, 2017 at 1:15 AM, Adrian Graham <wit...@binarydinosaurs.co.uk> wrote: > On 15/01/2017 14:38, "Tony Duell" <ard.p850...@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.
Basically, memory mapped I/O means having devices addressed as memory that perform I/O functions. You access them with the same load/store instructions that you use on real memory. On some processors (6502, 6800, 68000. PDP11, etc) that's all you have, there are no special I/O instructions. On others (8080, 8085, 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). 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. > 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 - Ah.... I can see what appear to be thick-film resistors on them (the black rectangles). Are there more conventional components on the underside? > http://www.binarydinosaurs.co.uk/STCexecutelboard.jpg > 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 Ah, I'd forgotten there was an SAA5070 on this board... > 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: > > http://www.binarydinosaurs.co.uk/STCExecutelBlownCaps.jpg > > 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. 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... If it is the drive I am thinking of, I have one somewhere, meaning I can look up the capacitors. But I would expect the thing to produce video without it. 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. -tony
