POST Cards

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

The Power-On Self Test (POST) in the 5170 motherboard outputs codes to I/O port 80h, and so the vast majority of POST cards will be monitoring port 80h.
Other machines started to output on different ports.
The following list is from the manual for my 'Micro 2000' made POST card:

(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

Therefore, some POST cards may have switches/jumpers to change which port is being monitored.

IBM 5150 (IBM PC)

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

Some XT clones might possibly output POST codes to port 80h.

IBM 5170 (IBM AT)

The POST codes that the 5170 motherboard generates are listed at here.