Marrow Design MM256K S-100 memory Board– Mystery Jumpers…

I purchased this board on Ebay since it looks like a good 256K board for a S-100 based computer.  Unfortunately I could not find any documentation for it.


After receiving it I noticed there was a single bank of jumpers.  What they were for I did not know but I suspected that they were for some sort of bank or extended address selection and perhaps some options.


This was marked as ‘J19’ on the board.  there are 6 jumpers out of 8 ‘positions’ – the top two (not seen above) were not selectable by jumper.


While I did not have the documentation – I did have a set of schematics from (a great repository of old documentation).


The section of these schematics which cover this set of jumper are as:




It looks like these jumpers are leading to some chip (U68) which I figured out to be a 8-bit equal comparator.  Comparing 2 sets of 8 bits and giving a true if they are equal.  The other side leads (eventually) to the A16 – A23 address pins of the bus.  The writing just above seems to indicate a ‘jumpered’ setting (in) is a zero and an unjumpered setting is a 1.


So my conclusion seems to indicate that these jumpers are used to select one of the 256K banks within the 16Mb [extended] address space of the S-100 bus.  Something which seems to conform this are some notes along the left hand of the same schematics:




U68 goes into the ‘A’ position (which it is – you can just make it out in the picture that it is to the left – the ‘A’ position) for Extended Addressing and the ‘B’ position for Bank select.