help with TRS-80 Model II?

From: Richard W. Schauer <>
Date: Sun Jan 11 02:31:55 1998

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.

> I assume the clock input is going high enough - from memory a TTL output
> needs a pull-up to drive a Z80.

CLOCK is swinging from 0.2 to 4.8v at 4MHz.

> You really need to be able to distinguish 'Low' from 'Tristate'. If
> you're using a 'scope, then pull the input to the Vcc (+5V) line through
> a resistor equal to the 'scopes input impedance (1M for most 'scopes, 10M
> if you're using a *10 probe) so that the trace floats at 2.5V and
> indicates highs and lows. Very few logic analysers (the HP LogicDart is
> an exception) have high/low/undefined inputs, alas.

Well, I did almost as you said. I had a 47k resistor laying handy on
the bench, so I used it. Part of the address bus is low (0.2 or 0.3v),
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.
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
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. (???)

> 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.

Richard Schauer
Received on Sun Jan 11 1998 - 02:31:55 GMT

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