Сheck with a multimeter that the A15 signal does not short circuit to ground (and VCC). Anyway, the problem is somewhere near the A15 signal.
I suggest at this point contaction the author of the board, either on his website or on vcfed (user pearce_jj)
I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O
On the back side of the board, the trace between pins 11 and 12 on your SN74F521 looks odd.
It's probably just flux or a JPEG compression artifact, but maybe check it out anyway. It's directly above the 8 on the back silkscreened documentation.
Thanks for your replies!
I repeated the tests in my VLSI board with both cards. Indeed there is a difference: In the VLSI it shows 64kb ROM, while in the ST62 its only 32kb. But the memory locations are the same.
No difference between both cards. This makes it very unlikely the issue is caused by bad soldering.
I also checked signals again but I didnt find something suspicious.
Maybe its a good idea to ask the author to take a look at this thread...
Sorry to hear you are having problems! This card is super simple so really the problem can only be the comparator or the pull ups.
I would suggest bench checking the comparitor input pins with a multimeter, with the card outside of the machine, testing from exposed comparitor IC leg on component side to vcc or ground, with the switches set, to ensure the pins are being held as expected. Then work back.
Also if the machine has an ISA bus speed setting, check it’s using an 800ns period for 8 bit cards.
OK I made some progress here in my ATARI-XT Mainboard.
I set the DIPs to C000, D000, E000, F000 in 64 kb mode and the ROM appears where it should! Therefore the card seems to be working.
I am very surprised it also shows up at F000:0 because there the Mainboard-ROM should be located...therefore the ISA-ROM seems to superseed the Mainboard-ROM?
As this seemed to be working, I removed the Mainboard-ROM and tried to boot via the Lo-Tech...but nothing.
For now I am convinced my Lo-tech is OK and the troubles were caused by some Mainboard settings I do not understand yet...
Thanks to all for your support!!!
If somebody managed to boot a Mainboard with the Lo-Tech-ROM please let me know...
Good stuff, which main board are you testing? It’s certainly possible in 5160, YMMV with others...
