IBM 5170  -  POST Code of DD


The Power-On Self Test (POST) in the IBM 5170 (IBM AT) generates POST codes.  Those POST codes can be viewed via the use of a of POST card (set to port 80h).

In the 5170, the POST code of DD indicates a RAM error in RAM bank 0.

The 'DD' code will be followed by either one or two codes, and be output in an endless repeating loop:
Example: DD,00,08,DD,00,08,DD,00,08,DD,00,08,DD,00,08 ...

Note that a RAM error does not necessarily mean that a RAM chip has failed.  It could be that a RAM chip has developed poor electrical contact with its socket, and a fix is simply to reseat the chip (wiggle the chip in its socket).


First
Code
Second
Code
Third
Code
  Bit at fault
DD 00 none  One of the two parity chips, PL or PH
DD 00 01  Bit 0 chip
DD 00 02  Bit 1 chip
DD 00 04  Bit 2 chip
DD 00 08  Bit 3 chip
DD 00 10  Bit 4 chip
DD 00 20  Bit 5 chip
DD 00 40  Bit 5 chip
DD 00 80  Bit 7 chip
DD 01 00  Bit 8 chip
DD 02 00  Bit 9 chip
DD 04 00  Bit 10 chip
DD 08 00  Bit 11 chip
DD 10 00  Bit 12 chip
DD 20 00  Bit 13 chip
DD 40 00  Bit 14 chip
DD 80 00  Bit 15 chip


Multiple bit failure

You will see variations of the above if the motherboard has multiple faulty RAM chips in bank 0.  Use the diagram at here to calculate the failing bits.


5170 motherboard of type 1

A type 1 motherboard has 36 RAM chips making up 2 banks of RAM.  (Photo)
Each bank consists of two rows of chips.
The motherboard has markings to indicate which is bank 0 and which is bank 1.  (Photo)
Bank 0: 16 data chips plus 2 parity chips.
Bank 1: 16 data chips plus 2 parity chips.

Details of the RAM chips used on a type 1 motherboard are at here


5170 motherboard of types 2 and 3

Type 2 and type 3 motherboards have 18 RAM chips making up 1 bank of RAM.  (Photo)
It might look like there are 2 banks, but there is only one (bank 0).
Bank 0: 16 data chips plus 2 parity chips.

Details of the RAM chips used on types 2 and 3 motherboards are at here