Weird ZX81 Behavior Cased By ROM

[1024MAK]

Yeah, I get the same values. Tested on a different emulator. And for what you are calling ROM 3 (649) on a real ZX81.

550 ROM checksum is 3424166000
622 ROM checksum is 3424958100
649 ROM checksum is 3424957100

Mark

[bwinkel67]

Ok, so testing the flaky ROM on another TS1000 with a different ULA (no backporch signal), it won't even boot up. Won't get to the cursor and sometimes throws garbage up on the screen. When put on the TS1000 with backporch signal, at least it behaves mostly normal. Again, can checksum it and it will work without RAM pack, but as soon as RAM pack installed, with ULA that doesn't support backporch, it won't even bring the machine up.

[1024MAK]

It may be that one of the ROM chip data bus output drivers is weak. With a RAM pack attached, there is more load on the data bus output drivers.

Not every board is the same, as although different chips are logically compatible, there may be slight differences in their electrical properties.

Mark

[bwinkel67]

It may be that one of the ROM chip data bus output drivers is weak. With a RAM pack attached, there is more load on the data bus output drivers.

Not every board is the same, as although different chips are logically compatible, there may be slight differences in their electrical properties.

Mark

Interesting, thank you. I've used that particular TS1000 for years now with little problem, but it was this little MSDOS simulator that brought it out:

msdos.p

(5.08 KiB) Downloaded 121 times

It heavily relies on INKEY$ as it has it in a somewhat tight loop if nothing else is pressed. Other software I ran would do a bunch of stuff in between, so maybe it was faulty from the start, or slowly failing as time went on. Perhaps some failures I attributed to RAM wobble. Of course, being the first to open up that TS1000, a fully working keyboard turned quickly into a brittle fest...those ribbon cables are fine if untouched but as soon as you wiggle them just a little, cracks appear quickly.

I ordered a replacement though a bit disappointed they don't make the US version since we don't say RUBOUT or NEW LINE in ours...I guess I'll have a hybrid UK/US version now :-/