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S100 - 1st? standardized PC bus. (1974-1975)

Called a back-pane.

Used a 100 pin connector.   

CPU was on separate card(s) which plugged into bus just like I/O cards.

Altair 8800 (Intel 8080 CPU). 1st use.

Early design did not really follow a protocol.

1st models used an 18v power supply which need to by pulled down 
to 5 volt and/or 12 volts.

Finally standardized (IEEE 696)  Used extensively in late 80 for CPM
style machines.