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ISA  POST  Cards


POST = Power-On Self Test

POST cards (for the ISA bus) appeared around the time of the IBM 5170 (IBM AT).

POST cards display the numerous "POST codes" that are output at various checkpoints of the Power-On Self Test (POST).  It is wrong to think of POST codes as error codes.  Think of them as a type of progress indicator.

For example.  I put an ISA POST card into a faulty IBM 5170 (one that has original IBM BIOS ROM's), and then power on the 5170.  I see the POST card displaying various numbers, and then the numbers stop changing, 08 being displayed.  Referring to the list at here, a list specific to the IBM 5170, I see that 08 corresponds to the 5170's POST progressing as far as the 'DMA page register test'.

The POST within the IBM BIOS of an IBM 5170 motherboard outputs its codes to I/O port 80h, and so POST cards that have an ISA bus connector are expected, at the least, to be monitoring port 80h.
The POST of certain (certain, not all) clones of the IBM 5170 also send codes to I/O port 80h.
Later, the IBM 5162 was released, and its POST also sends codes to I/O port 80h.

The POST of certain other machines output on a different port.  The following list is from the manual for my 'Micro 2000' made POST card:

Port (in hex)	Machine Type
80	[standard usage]
84	All Compaq computers
90	PS/2 models 25 and 30
300	Award BIOS
680	Computers with Microchannel bus

The following list is from the book, The PC Engineer's Reference Book - Volume 1:

Architecture	Typical Computer	Port (in hex)
PC	PC	none
ISA	XT	60
	AT	80
	PS/2 25,30	90, 190
MCA	PS/2 50 up	680, 3BC
EISA	none	none

Below, the particular example of a vintage POST card that is pictured, shows switches that are used to control which I/O port is being monitored.

Modern POST cards, such as the particular example pictured below, have no port setting switches.  Presumably, they monitor multiple ports simultaneously.  If so, which ports?  Hopefully, the manual supplied with the card contains such information.


IBM 5150 (IBM PC)

The POST within the IBM BIOS of an IBM 5150 motherboard does not output POST codes.
Any numbers that you may see displayed by a POST card are not POST codes; they will be the result of something else.


IBM 5155 (IBM Portable PC)

The IBM 5155 contains an early 5160 motherboard.
Read the section below that covers the 5160.


IBM 5160 (IBM XT)

The POST within the IBM BIOS of an IBM 5160 motherboard outputs only a few POST codes, and does so to port 60h.
Those codes are the four 'checkpoints' listed here.
I have yet to hear of a POST card that monitors port 60h.

The BIOS' of some XT clones might possibly output POST codes to port 80h.


IBM 5162 (IBM XT Model 286)

The POST codes that the POST of the IBM BIOS of an IBM 5162 motherboard generates are listed at here.


IBM 5170 (IBM AT)

The POST codes that the POST of the IBM BIOS of an IBM 5170 motherboard generates are listed at here.

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Example of a modern POST card


I have verified that this particular model monitors I/O port 80h on its ISA bus connector.  I do not know what other ports it may monitor (the manual supplied being of no help in that regard).

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Example of a vintage POST card

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Example of an antique POST card