in honor of the IBM PC's 45th anniversary today, here is the original prototype motherboard!
which side is your favorite?
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What are those white parallel lines just above the power connector? I thought dip switches until I zoomed in
@elithebearded @tubetime I think they’re resistor packs for bus pull-up or pulldown. They’re right next to the Beckman DIP rpacks, and there’s a similar DIP header with assorted resistors nearby too
@Cdespinosa @elithebearded @tubetime
It's two octal rocker DIP switches, and one fully bussed resistor network, which presumably supplies the pull-up for 15 of the 16 switches.
@tubetime yeah those decoupling caps stretched out along the ICs are just *chefs kiss*
@jpm @tubetime Textbooks say you should locate decoupling capacitors "as close as possible to the IC", so sticking them directly on top is a stroke of genius!
But, what really gets me is how you can see the prototype developed in waves: first, they soldered a decoupling capacitor on top of most of the chips. Then, they returned to add parallel decoupling capacitors to the interrupt controller chips, and to the chips between the 8086/8088 and the ROM (chache memory?). Then, in the wire-wrap nest on the other side, they sprinkled two further types of caps on the RAM sockets to broaden the decoupling network.
Incidentally, this motherboard can accommodate 64 kilobytes of RAM. If you count the sockets, you can see they took care to add an extra RAM chip to each 32K bank for parity.
Amusingly, the majority of those RAM sockets appear unpopulated. So do the 8087 and 8088 sockets. I imagine an engineer scavenged a few components to fix another board, as tradition dictates.
