IBM PC hacking

[Tony Duell]

> 
> 
> 
>  > The problem with those diagnostics (and most built-in diagnostics) is
>  > that a fairly large part of the machine has to be operational (at
>  > least the CPU, some memory, ROM, video) for them to be at all useful.
> 
>  > -tony
> 
> 
> One of the lesser known features of the PCjr was the manufacturing test 
> mode.  I think output was via the ports on the back.  (Not the famous 

Accroding to the BIOS source listings, some test outputs are written to 
ports of the 8255 chip on the mainboard. I think the only way to get to 
them is with a text clip on that IC. These tests are basically the same 
as the POST tests in a PC, etc.

I've not had time to properly read the BIOS source, but it appears there 
was a burn-in test mode which you got by grounding all 4 joystick button 
lines and powering the machine up.

> 'L' port though, which appears to have no use.)  The manufacturing test 

According to the schematiocs, there are 5 pins on the L connector. 1 is 
ground, the other 4 are not connected 'spares'. 

-tony