SpidersWeb
Veteran Member
So I had a spare Turbo 8088 style board laying about - just another Taiwan special.
Anyway in the process of building an XT with it, I noticed it had 512KB of RAM on board but the RAM test only showed 256KB.
This isn't the end of the world because in the end a jumper switch + an AST Six Pak Plus, it now has 640KB.
But I'm mostly curious about what I missed / don't understand here when trying to diagnose the bad chip on the board itself.
Supersoft Diagnostics ROM showed bit 0 of bank 1 as bad.
- replaced the chip
- swapped the chips around in the bank
- replaced the socket
No change from all of these.
- tested CAS was linked with appropriate chips
- tested RAS was linked with appropriate chips
- replaced DRAM in bank 0 bit 0 (just in case it was holding the data line high or low)
- tested all address lines had continuity
- tested Data line had continuity (and all other appropriate pins)
No problems detected or changes in results.
If there was a problem with CAS, I'd expect a string of chip failures. Ditto for RAS.
If there was a problem with the data line, I'd expect all chips to fail.
If there was a problem with addressing, I'd expect it to not be limited to bit 0.
After adding the AST card (after all of the tests above):
When the on-board memory was not limited to 256KB, it also caused memory errors on the AST card including right through the 512KB to 640KB region - and with random bits and almost random locations. (I expected errors 256-512)
When the on-board memory was limited to bank 0 only, the memory test passed all the way up to 640KB.
Just has me scratching my head.
Anyway in the process of building an XT with it, I noticed it had 512KB of RAM on board but the RAM test only showed 256KB.
This isn't the end of the world because in the end a jumper switch + an AST Six Pak Plus, it now has 640KB.
But I'm mostly curious about what I missed / don't understand here when trying to diagnose the bad chip on the board itself.
Supersoft Diagnostics ROM showed bit 0 of bank 1 as bad.
- replaced the chip
- swapped the chips around in the bank
- replaced the socket
No change from all of these.
- tested CAS was linked with appropriate chips
- tested RAS was linked with appropriate chips
- replaced DRAM in bank 0 bit 0 (just in case it was holding the data line high or low)
- tested all address lines had continuity
- tested Data line had continuity (and all other appropriate pins)
No problems detected or changes in results.
If there was a problem with CAS, I'd expect a string of chip failures. Ditto for RAS.
If there was a problem with the data line, I'd expect all chips to fail.
If there was a problem with addressing, I'd expect it to not be limited to bit 0.
After adding the AST card (after all of the tests above):
When the on-board memory was not limited to 256KB, it also caused memory errors on the AST card including right through the 512KB to 640KB region - and with random bits and almost random locations. (I expected errors 256-512)
When the on-board memory was limited to bank 0 only, the memory test passed all the way up to 640KB.
Just has me scratching my head.
