Great Hierophant
Veteran Member
Am I right about the following, or is there something else that needs be taken into consideration :
I suspect that there are far more 6MHz IBM AT machines and boards out there than 8MHz AT or 6MHz XT/286 boards. An inexpensive way to make them faster is to upgrade them to 8MHz.
The IBM AT is modular with two clocks, one for the CPU (12MHz divided by 2) and one for the rest of the I/O chips (14.318180MHz divided).
In order to obtain a higher clock speed, you must switch out the crystal for a 16MHz one of the same type. Additionally you must make sure that your various components are rated for the higher speed :
CPU must be an 80286-8 or better and be of the PGA type
RAM must be 125ns or faster (shouldn't be an issue, but you may need to piggyback two chips if you have less than 512K and have the first version of the motherboard or use a memory board to backfill the RAM).
DMA chips must be 8237A-5 (they should be but you need to check since DMA runs at 4MHz in an 8MHz machine and 3MHz in a 6MHz machine and the stuck 8237A is only rated for 3MHz).
Now that the system is comparatively fast, that BIOS needs some work so additional drive types, the Model M keyboard and 3.5" drives are properly supported. You could burn the 11/15/85 BIOS (for the 8MHz AT) and use that. In fact, if you have the 06/10/85 BIOS, you need to do this or your system will fail the speed check.
Suppose you had an XT/286 system, you could do the exact same thing, except it also has a speed check for 6MHz. I do not know whether the AT 11/15/85 BIOS will work in the system due to the lack of wait state hardware and other minor differences, so you may need to burn a generic AT compatible BIOS.
I suspect that there are far more 6MHz IBM AT machines and boards out there than 8MHz AT or 6MHz XT/286 boards. An inexpensive way to make them faster is to upgrade them to 8MHz.
The IBM AT is modular with two clocks, one for the CPU (12MHz divided by 2) and one for the rest of the I/O chips (14.318180MHz divided).
In order to obtain a higher clock speed, you must switch out the crystal for a 16MHz one of the same type. Additionally you must make sure that your various components are rated for the higher speed :
CPU must be an 80286-8 or better and be of the PGA type
RAM must be 125ns or faster (shouldn't be an issue, but you may need to piggyback two chips if you have less than 512K and have the first version of the motherboard or use a memory board to backfill the RAM).
DMA chips must be 8237A-5 (they should be but you need to check since DMA runs at 4MHz in an 8MHz machine and 3MHz in a 6MHz machine and the stuck 8237A is only rated for 3MHz).
Now that the system is comparatively fast, that BIOS needs some work so additional drive types, the Model M keyboard and 3.5" drives are properly supported. You could burn the 11/15/85 BIOS (for the 8MHz AT) and use that. In fact, if you have the 06/10/85 BIOS, you need to do this or your system will fail the speed check.
Suppose you had an XT/286 system, you could do the exact same thing, except it also has a speed check for 6MHz. I do not know whether the AT 11/15/85 BIOS will work in the system due to the lack of wait state hardware and other minor differences, so you may need to burn a generic AT compatible BIOS.