ard at p850ug1.demon.co.uk
Mon Feb 29 12:36:57 CST 2016
> FWIW, the IBM term for "motherboard" was "planar", at least in the era of
> the PC, PC/XT, PC/AT, etc.
But IBM (at that time) also used the term 'Planar' for 'backplane'. The backplane
in the 5161 expansion unit  is labelled 'I/O Planar' or something very similar
in the silkscreen.
 This is what I would term a backplane rather than a motherboard. It is a
passive bus. The only real electronics on it is a 14.3..MHz oscillator to provide
the one signal they didn't take (with good reason) from the machine the
expanison unit was connected to.
I agree with the distincton drawn by others. a 'motherboard' contains significant
parts of the computer circuitry, whereas a 'backplane' is either entirely passive or
contains simple bits of circuitry like buffers, address decoders, clock, reset logic,
In the HP150, the boards slid in from the rear and connected to a PCB at the front
of the case containing the printer interface and bus connectors. This (owing to
its position) is called the 'frontplane' in the HP techincal manuals.
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