Interesting HP-65 card reader failure mode

[John Garza (3665)]

OK, I put the old Teenix CPU in HP65 #1333A06439 (a known good unit).

Writing then reading a program still inserts the STO 4, same as a reading a prerecorded card.

However I did notice something unusual. Hitting a RTN in RUN mode, then switching to W/PRGM the mysterious STO 4 disappears.

Procedure is:
1) load card in RUN mode
2) immediately switch to W/PRGM see 33 04
3) hit SST and see the first instruction from the card
4) switch to RUN
5) hit RTN
6) switch to W/PRGM and see 00 00
7) hit SST and see the first instruction from the card

Looks like the 33 04 is briefly taking the place of the normal 00 00 top of memory marker.
If that's the case, then it's not actually in program memory.
Likely a harmless artifact.

As for the faulty machine, HP65 #1333A09948, it cannot read a card it records. I get flashing zeroes and NOPs in memory.


-J