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I have used the NOP's in eprom technique to test the basics of a
system before now, you can scope the address lines to make sure that
the pattern A14 is half the width of A15, and A13 is half of A14,
etc.
 some of the basic checks may show some oddities.  but depending upon
the rest of the circuit you may not get much further. Definitely worth
a try . 

	-Jim
 On Thu 22/07/10 4:14 PM , "Roger Weichert" rweicher@bigpond.net.au
sent:
  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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