[Simh] Unibus XOR Tester

[Don North]

Tim Shoppa wrote:
> Walter writes:
>   
>> when looking through a few listings of XXDP tests, I saw code
>> sections mentioning a
>>     
>> 	XOR Tester
>>     
>> This was obviously a Unibus device at address 177060.
>>     
> A internal DEC memo mentions "PDP-11 Enginerring (Larry Condon)
> has developed an XOR tester for small peripheral controllers that
> plug into DD11 backpanels."
>
>   http://www.bitsavers.org/pdf/dec/pdp11/memos/731015_Controllers_Cables_Packaging.pdf
>
> This was the realization I think that a SPC could fit into a single slot instead
> of requiring its own little backplane per device.
>   

DEC used 'XOR testers' in their module manufacturing test process. 
Basically they would send the same set of test patterns into the inputs 
of a known-good ('gold') module and an unknown ('DUT') module in 
parallel, and then 'xor' the outputs. Any differences indicated the DUT 
was not functioning correctly compared to the gold-standard module.

Most of these testers were built from PDP-8s or PDP-15s, and later 
PDP-11s. DEC developed special test interface boards (which later became 
the standard digital input/output modules). Ultimately these testers 
went away as the more flexible GENRAD class of testers became available.

Don