HP27 Restoration (Lots of Corrosion)

[teenix]

Attached is scope shots of a HP-25

Reset - 50mS/div, a bit noisy, but my setup probably not ideal.
Sync - 500us/div
ISA - 50uS/div

The sync is as is because of the code it is running during the key wait loop. Sync stays LO after an IF instruction in the code. This tells the ACT that the code value itself is a ROM address.

displ5: 0 -> s3 <<< sync HI
if s3 = 0 <<< sync HI
then go to dsst4 <<< sync LO, goto ignored here as S3 = 1
if s15 = 0 <<< sync HI
then go to displ5 <<< sync LO, got executed as S15 = 0 (no keypress)

The ISA bus is showing the code executing for the above.


For the HP-27 it will be executing...

if s15 = 0 <<< sync = HI
then go to $08A <<< sync = LO, got executed as S15 = 0 (no keypress)

so the Sync line will be alternating HI and LO for each instruction.

cheers

Tony