'motherboard' etymology

[Peter Cetinski]

> Does anyone know the origins of the term 'motherboard'?
> 
> I've always associated it with computers and assumed that it started 
> appearing somewhere around 1980, with the fading out of passive backplane 
> systems and arrival of machines which put more functionality onto a 'core' 
> PCB into which other cards were plugged. I don't recall ever seeing it used 
> when referencing earlier big iron, but maybe I've just missed it.
> 

Here’s a little more muddying of the “motherboard” term.  When Tandy introduced the Model II in 1980, they named what is essentially a passive backplane as the “Motherboard” (note the capital M).  This was the main bus for all of the Z80 functions that were provided on plug-in cards, including the CPU card, the floppy controller card, the memory cards and the video/keyboard card.  Then, when they introduced the Model 12/16B a few years later, they had consolidated almost all of the Z80 architecture onto a single board which they referred to as the "Main Logic" board.  The 16B provided a variant of the original passive backplane, to support various additional cards, that plugged into the Main Logic board and they still referred to this as the Motherboard.  So, even though the Main Logic board was what we think of as a motherboard today, the kept the “Motherboard” naming convention for the backplane.