How to read parallel-port POST diagnostic codes?
There are many cheap diagnostic cards available on ebay that read the codes output to port 80h during POST and display them on a standard 7-seg LED. But what if you have a board that outputs to 378h (ie. LPT)?
I was trying to help someone with a dead AT&T 6300 (it boots up to a "0" on the monitor before dying) by disassembling the 1.43 ROM BIOS (using the 6300 Plus BIOS as a guide, as I'm not aware of the 1.43 BIOS source published anywhere). I was surprised to see that there are clear "Check Points" output at various points in the POST code:
(The codes range from 40h to 4Fh, which I'm assuming are common printer control codes that don't print anything, as I don't remember my 6300 printing characters every time it booted.)
seg000:DB8F i_powerup: ; CODE XREF: seg000:E05Bj
seg000:DB8F ; seg000:EA73j
seg000:DB90 mov al, 40h ; '@' ; checkpoint #0
seg000:DB92 mov dx, 378h
seg000:DB95 out dx, al ; Printer Data Latch:
seg000:DB95 ; send byte to printer
How would someone read these codes? Is there are relatively straightforward way to, for example, wire up a cable from the 6300 to something else, or is there an easier method I'm missing?
Offering a bounty for:
- Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)