Depending on needed fan-out you can improvise a NOP forcer by removing the EPROM
from its socket and soldering 8 1N4148 diodes between the /OE pin thereof and
each of the data bus pins on a thin pinned socket. Then put the prepared jog
socket in the EPROM socket and run the system. Be sure to disconnect all time
critical hardware. Most systems do not use both /CE and /OE, /OE is more likely
to be used.

Note that this may create a very high fan-out requirement situation for the /OE
driver on the board, depending on what is on the board connected to the EPROM
local data bus. In this case a hex open collector output chip can be used to
create the circuit on the EPROM socket. You can use 2 pcs 7407 to make the
drivers in this case, again driven by /OE. The Z80 opcode for NOP is 00h so
driving /OE low will pull all 7407 outputs low and they will hold the data bus
that way, thus forcing 00h onto it. You can also use an octal latch or buffer
with tristate outputs, wire its data inputs to L and connect its /OE to the
EPROM /OE and its data outputs to the EPROM data pins.

Depending on how the EPROM is decoded this may cause the Z80 to roll through the
entire address space or only through a portion of it. It is possible to
complicate the circuit by detecting the highest address being issued and force a
RST instruction on the bus in that case, which will cause the Z80 to prectically
loop to near the beginning of the EPROM. RST 38h is opcode FFh in IM1.

Assuming 4MHz clock on Z80 it will run through its 64k address space at a rate
of about 15Hz, fast enough that you can scope the address decoding logic.

hope this helps,

-- Peter


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