Intel MDS Series II IOC question

Dave Mabry dmabry at
Sat Sep 3 23:30:58 CDT 2005

Tony Duell wrote:

>I am certainly interested, even though I don't have one of these machines.
Today in a failed attempt to repair the IOC board, I resurrected some 
ancient hardware.  The IOC board is the back panel board on an Intel MDS 
Series II that handles most of the I/O of the system.  It does a lot!  
It is based on a 2.4MHz 8080A cpu.

Well, I broke out an Intel ICE-80 and hooked it up.  This is the first 
In-circuit emulator that I know of, perhaps the first ever.  It is quite 
primitive, but it can do a lot in the hands of a skilled operator.  If I 
ever find such an operator, maybe I will be able to fix this damned IOC 

One of the problems I encountered with the emulator is because the RAM 
in the target system is dynamic.  I think that when the emulation 
"breaks" from a breakpoint it seems that the RAM loses its contents.  
Perhaps the refresh circuitry relys on a clocking cpu.  But it does look 
like I get inconsistent results reading the ROM on the IOC board.  So I 
will now look at the buffer chips and maybe the address decoding.  I 
tried to map the execution memory to the host MDS (that is the machine 
that is hosting the ICE-80), but since that slows execution and there 
are some software timing loops that didn't work very well.

I am making progress, slowly.  And putting two of these beasts together 
(one working one for the ICE-80 host, and the "target" system that needs 
to be fixed) isn't trivial.  They weigh about 100 pounds each!!!

It was a comical scene, though, with two MDS Series II systems side by 
side.  I thought my table was going to colapse and the lights dimmed 
when I turned them on.  ;)

Stay tuned...I have some more disassembling to do.


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