help with TRS-80 Model II?

From: Christopher Denham <cdenham_at_tgis.co.uk>
Date: Sun Jan 11 17:22:43 1998

On Sun, 11 Jan 1998 17:03:28 +0000 (GMT), you wrote:

>>
>> On Sun, 11 Jan 1998, Tony Duell wrote:
>>
>> ........
>> > What is the reset pin doing at the Z80? And what is the WAIT* pin doing.
>> > I wonder if the CPU is either stuck in the reset state, or in a wait
>> > state.
>>
>> RESET* is low for about 500 ms after power-on, then goes high. WAIT* is
>> always high.
>>
>> > I'm pretty sure that if the CPU is halted, it continues to produce
>> > refresh addresses, so you'd see some activity on the address lines and on
>> > RFSH*. Can't hurt to check, though.
>>
>> RFSH* is at a solid 1.4v, and doesn't move with a pull-up.
>
>Wait a second... It's at a constant, DC 1.4V? Are you sure?
>
>You're not missing a power line, are you? I suppose a low supply rail on
>chips driven by this signal could cause some strange happenings. Check
>all the outputs from the power supply with a 'scope. Apart from that, I
>am confused. There's no way a TTL output should be stuck at that voltage.
>
>
>> part is tri-stated. Data bus lines D0-D2 and D7 are at a firm 1.4v. D3
>> is tri-stated and can be pulled to 5v. D4-D6 are low and pull-up to 1.4v.
>
>Kersqueeble... What on earth is going on here.
>
>> Now, here's where it gets interesting. NMI* had an irregular pulse train
>> coming into it- at the same time the display was flickering. I shut off
>
>There may well be an interrupt from the display system to the CPU - a
>sort of poor-mans heartbeat for the real time clock.
>
>
>> and turned on again, and the display was blank and NMI* was high. INT*,
>> WAIT*, BUSRQ*, and RESET* are high, BUSAK* is low, and HALT*, MREQ*,
>> IORQ*, RD*, WR*, M1*, and RFSH* are all at a solid 1.4v. (???)
>
>That 1.4V worries me. What about the buffers on these signals? Are they
>getting the 5V power?
>
>>
>> > Do you have a schematic of the gate-array version of the M4 CPU board?
>> > I've got an M4 here with said board, and it's the only part of the system
>> > that's not covered in the M3 Techref (which I also have), so I am looking
>> > for a copy/scanned in version of the diagram.
>> >
>> > I would buy a Model 4 technical manual, but Tandy in the UK are totally
>> > clueless and assured me that there was no such machine as a 'TRS-80' :-)
>>
>> I have a 6-page schematic for the CPU, and it shows 4 PAL's. It is dated
>> 23 Feb 1983. Contact me privately and we'll work out how to get it from
>> here to there.
>
>Alas, I think that's the older version of the CPU board. Is there a 6845
>on it? Later versions, like the one in my machine, have a custom Tandy 40
>pin gate array alongside the CPU which contains most of the video
>circuitry, etc.
>
>On the other hand, it's a lot better than nothing, so if you could make a
>copy of it, we can come to some arrangement about getting it to me.
>
>
>>
>> Richard Schauer
>> rws_at_ais.net
>
I have a TRS-80 MODEL II TECHNICAL REFERENCE HANDBOOK
Catalog Number 26-4921 dated 1980 and a flatbed scanner so any pages
you would like to see i could email them to you direct .........

Cheers Chris Denham cdenham_at_tgis.co.uk
Received on Sun Jan 11 1998 - 17:22:43 GMT