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Thread: How to read parallel-port POST diagnostic codes?

  1. #1
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    Default 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:

    Code:
    seg000:DB8F i_powerup:                              ; CODE XREF: seg000:E05Bj
    seg000:DB8F                                         ; seg000:EA73j
    seg000:DB8F                 cli
    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
    (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.)

    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?
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  2. #2
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    On one of my systems, the parallel port is connected (on board) to an 8-LED DIP display. You just read the code right off the board. You could do the same thing with a bunch of LEDs soldered onto a DB25M plug.

  3. #3

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    Quote Originally Posted by Trixter View Post
    (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.)
    40h to 4Fh are just common ASCII codes from '@' (as depicted in your code) followed by 'A' up to 'O' (check any ASCII table)

    If there is no strobe puls generated the printer will ignore those bytes but you can check them on the data pins of the LPT port.

  4. #4

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    Assuming you don't mind spending a bit of cash and waiting a month for delivery from China why not just get a POST reader for a LPT? They are commonly available on eBay.
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  5. #5

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    Quote Originally Posted by gertk View Post
    40h to 4Fh are just common ASCII codes from '@' (as depicted in your code) followed by 'A' up to 'O' (check any ASCII table)

    If there is no strobe puls generated the printer will ignore those bytes but you can check them on the data pins of the LPT port.
    In which case installing a switch to pull the strobe line will cause the printer to print the status characters.
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  6. #6
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    Quote Originally Posted by Shadow Lord View Post
    Assuming you don't mind spending a bit of cash and waiting a month for delivery from China why not just get a POST reader for a LPT? They are commonly available on eBay.
    I did not know these existed! I only knew about the PCI/ISA interface ones. Thanks, this will do the trick nicely.
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    Many of the Olivetti PCs are using parallel port to output POST diagnostics. We can say, all of them which are using Olivetti own BIOS. That also includes AT&T 6300, Xerox 6060 and many PCs from Triumph Adler while that company was owned by Olivetti. It should be all Olivetti PCs and laptops up to about 1992. Then they started to use AMI BIOS on their own boards or buying boards from elsewhere, there a classic ISA/PCI POST diag card works fine.

    Unfortunatelly I could not find anywhere a POST code list for such Olivetti PCs until yet. But it is still usefull, you can see this way if the processor does something sensefull or not before display comes up.

    But you can buy ready made test modules from ebay which are working just fine on Olivetti PCs for just a few bucks.

    Example, it's also available from vendors in US, Europe or China: http://www.ebay.com/itm/Motherboard-...wAAOxyiOxR1PY6

    It has USB port, just for power supply. But on Olivetti PC not needed, the PC powers this module, I have tested it already and it works fine.

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    I wonder if you can't peruse the AT&T 6300 Plus BIOS listing and dig the codes and their meanings out of there.

    I do know that the Plus was an 80286 system, but I suspect that the basic POST codes are either very similar or the same.

  9. #9
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    6300 Plus is the same as Olivetti M240: 8086-10 XT on size shrinked mainboard and nicely restyled M24 chassis.

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    It's really odd to see the parallel port POST analyzer being sold on eBay. How many current desktops and laptops even have a traditional printer port on them?

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