A & A Scales PT300, Service Manual, Page: 20 / 52

PT300, Service Manual
Rev B, October, 2000
19
Tests, Waveforms and
Voltage measurements
Blank EPROM test.
One of the most challenging problems is finding a problem in the
"microprocessor kernel," that is, the RAM, ROM, microprocessor and related
circuitry. The hardest part is that everything is tied together, and if you cut it
apart, nothing works. The best way to tackle this problem is to run a VERY
simple test program. This allows you to determine that the microprocessor is
working first. Often, when the machine is "almost" working correctly, the
symptoms help you to find any remaining problems.
On a Z80 style microprocessor, the simplest test program is a BLANK EPROM.
If used properly, this program will find any shorted address lines, any shorted or
open data lines, any bad read, write, or chip select lines. It only uses the
microprocessor, ROM, and chip selector IC.
A blank EPROM returns 0FFH as the data. A quick look at the Z80 instruction
set shows this to be a short call to location 38H in memory. The microprocessor
pushes the current address location on wherever the stack pointer is pointing.
As the test runs the stack pointer decrements through every location in memory.
At location 38H the microprocessor reads another 0FFH. This repeats
indefinitely. Let’s examine this program on a cycle by cycle basis.