Service manual

PT300, Service Manual

Rev B, October, 2000

CLK

RFSH

_____

0038h

RFSH

See Below

0FFh

039h

00h

Figure 7 Blank EEPROM Test Waveforms

For the first three clock pulses the Z80 reads 0FFH from location 038H. As part

of an instruction fetch the Z80 does a memory refresh cycle. We do not use this

for anything, but its there. Then the Z80 writes out the address 0039 on the

stack. Note that this takes two write cycles. Each time the Z80 writes this out

the stack address changes so the address lines will count through all address.

The address lines can be examined one at a time. With practice you can verify

that these signals are correct in about two minutes. What you are looking for is

a characteristic pattern, with no "funny" voltage levels. A shorted line will be

unable to reach the low voltage levels. You may have to fiddle with the trigger

controls to get a stable display.