As soon as a system is built, it seems that I will already will want to expand its capabilities. So from the start the SBC-85 systems was built with an expansion bus and plans for a backplane. The SBC-85 Backplane is nothing more than a four-slot with power inlet (using the same barrel connector and reversed bias diode for polarity protection). After laying it out i did decide to go back and add in headers for all of the bus lines to make it easier to connect a scope or logic analyzer.
While probably not necessary, there is pull-up/pull-down terminators on each end of the bus on the clock line. A larger than necessary SIP was used with the unused terminators brought out to little vias so they can be patched as required. Finally, all of the bus lines are connected including the unused ‘reserved’ lines in case someone makes some boards that need a local board-to-board bus.
