IBM 5170  -  POST Code of DD


IMPORTANT:   The following assumes that your IBM 5170 motherboard is fitted with IBM 5170 BIOS ROM's (i.e. not swapped out for non-IBM ones).
Non-IBM BIOS' probably don't do what is shown in this web page.


The Power-On Self Test (POST) within the IBM BIOS of an IBM 5170 (IBM AT) generates POST codes.  Those POST codes can be viewed via the use of a POST card that monitors port 80h.

For the 5170, the POST code of DD indicates an error within the first 64 KB of motherboard 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.
For example, 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).
For example, part of the motherboard's RAM support circuitry has failed.


First
Code		 Second
Code		 Third
Code		   Bit in bank 0 at fault
DD 00 none  One (or possibly both) of the two parity chips, PL or PH, in bank 0
DD 00 01  Bit 0 chip, in bank 0
DD 00 02  Bit 1 chip, in bank 0
DD 00 04  Bit 2 chip, in bank 0
DD 00 08  Bit 3 chip, in bank 0
DD 00 10  Bit 4 chip, in bank 0
DD 00 20  Bit 5 chip, in bank 0
DD 00 40  Bit 6 chip, in bank 0
DD 00 80  Bit 7 chip, in bank 0
DD 01 00  Bit 8 chip, in bank 0
DD 02 00  Bit 9 chip, in bank 0
DD 04 00  Bit 10 chip, in bank 0
DD 08 00  Bit 11 chip, in bank 0
DD 10 00  Bit 12 chip, in bank 0
DD 20 00  Bit 13 chip, in bank 0
DD 40 00  Bit 14 chip, in bank 0
DD 80 00  Bit 15 chip, in bank 0



Multiple bit failure

You will see variations of the above if there are multiple bits in bank 0 that have failed.
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 1: 16 data chips plus 2 parity chips.
Bank 0: 16 data chips plus 2 parity chips.

Click here for a diagram that shows the bank and bit layout on the type 1 motherboard.

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.

Click here for a diagram that shows bit layout of bank 0 on the types 2 and 3 motherboards.

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