RAM Error Troubleshooting (1985)

Steve Nelson 5/29/85

A malfunctioning RAM chip will normally produce an error code that can be used to determine which of the chips is bad. There are several things needed to locate the bad chip

First determine whether your error came from the system board or a memory expansion unit. This is indicated by the diagnostics program showing a 3-digit error, 1xx or 2xx (the second and third digits don't matter yet). This is followed by a 4-digit number that will be used in a moment. System board errors start with 1 and expansion board errors start with 2. The following refers only to the errors in system memory, since expansion cards vary in layout.

Determine which type of system board you have (64K or 256K), then use the corresponding table below. The 64K boards usually have four rows of chips in the front left corner that end with 16 (e.g. 4116), while the 256K boards have chips that end with 64 (e.g. 4164, 4864, depending on the manufacturer). Make sure that you are trying to replace the bad chip with the correct new one

Now look at the 4-digit code that was given. The first two digits represent the "row" that the chip is in, while the second two tell how far down the row the chip is. The rows are labelled BANK0, BANK1, BANK2, and BANK3 on the circuit board at the far left edge. 

"A $3000 system board is not the place to brush up on your soldering techniques" (in 1985 they cost this much)

Some PC's will have BANK3 nearest the front edge of the computer and BANK0 at the rear, while some compatibles reverse this order to keep
things interesting. For that reason, double check the location of BANK0 on the board before proceeding. (If it is closer to the back, then continue, otherwise use a mirror.)

For 64K boards:

Second 2 Digits

First 2 Digits

For 256K boards:

Second 2 Digits

First 2 Digits

In general, expansion boards are arranged so that the rows run from the back of the computer to the front on the card, and the positions in the row run from the bottom to top. Therefore, you can try to figure out which chip is bad by using some hexadecimal arithmetic and lots of lucky guesses

The chip represented by 00 for the second 2 digits refer to the parity bit in the byte. This is either the top or bottom chip in the row, again depending on the manufacturer. On at least one AST Six-Pak, the parity bit is the located in the chip at the bottom of the board. The banks on the expansion board will start with error codes 2xx 4000 in a 256K system board computer and go as high as 2xx 9xxx with 640K of RAM installed. 64K machines start at 2xx 1000 on the expansion boards