Message-ID: <009001cb29b0$9607fd40$0100000a@roger29qgfmb4o>
From: "Roger Weichert" <rweicher@bigpond.net.au>
To: "Microcontroller discussion list - Public." <piclist@MIT.EDU>
Subject: [EE] Faultfinding Z80 based circuits
Date: Fri, 23 Jul 2010 00:44:31 +0930
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Hi guys,

I have a new question for today. I am working on a Z80 based system where 
the micro is not running properly.

The device is a 25 year old RFID reader which has a Z80 CPU, PIO, SIO, CTC, 
eprom and ram  ...  a whole 2k of each  :) .. plus a heap of additional 
basic circuitry.

To get the 'why' question out of the way  ... replacement units are US$3600 
and I've fixed a dozen already, which helps pay the bills.

Anyway, this one has a good supply, the clock signal is nice and clean, the 
Reset is not being held on  ...  and the only socketed device, the 2716 
eprom is ok.

The CRO shows signal on the data lines and the address lines (although not 
the same as a known good one), but nothing on the RD, IROQ, WR lines etc. 
The HALT and WAIT inputs are not being clamped.

I do have a good one to compare with, and I do have a range of spare chips 
from my 'playing' 30 years ago, but I'm not all that keen to start replacing 
40 pin chips willy nilly, especially seeing they're not in sockets.

I vaguely recall a fault finding technique of programming an eprom with a 
nop? to make the cpu cycle around, but I dont know how useful that would be 
with all the other chips in circuit and not easily removeable.

I can imagine a really useful diagnostic system by making up an eprom with 
an appropriate program in it, but I have no circuit of this thing to be able 
to work out just how each chip talks to each other

Does anyone have any suggestions for techniques for narrowing down this sort 
of fault? Thanks.

Regards,   Roger 

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