'motherboard' etymology

But IBM (at that time) also used the term 'Planar' for 'backplane'. The backplane in the 5161 expansion unit [1] is labelled 'I/O Planar' or something very similar in the silkscreen. [1] 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, etc. 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. -tony