Xerox 820-II troubleshooting - need advice

Tony Duell ard at
Mon Apr 27 13:15:28 CDT 2009

> Fortunately, it turns out to have been much simpler.  The 820-II firmware 
> apparently expects the FDC controller to be present during init.  To keep 
> things simple, I had the daughterboard unplugged.
> Once I smartened up and plugged the controller in, it started responding 
> to the keyboard.  I can only assume it resets the controller at power up 
> and is waiting for some sort of state change or interrupt.  Without the 
> board present it was just waiting forever and never getting to the stage 
> where it would acknowledge character input from the keyboard.

This reminds me in a way of a problem I had with a Whitechapel MG1 (A 
32016-based workstation). It flashed the error LED at power-on, reporting 
a multi-bit DRAM error. 

TO gcut a very long story short, after battling throuhg the memory 
arbitration logic, I discovered there was actually nothing wrong. The 
mainboard has 512K of RAM on it, the boot ROM needs (and tests) 1.5M. 
After plugging in the 2 RAM expansion boards I'd removed 'to keep things 
simple' (:-)), it worked fine. This is not documented in any of the 
manuals I have.

> Nice when things turn out this easy.

In my case it wasn't so simple. It took me a day to go through and check 
all the logic. It's a complcated machine...


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