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IBM PS2 Model 50 Schematics
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== What it settles == '''Sheet 11, REAL TIME CLOCK, settles the Model 50's clock circuit at component level.''' The RTC is '''ZM47, a Motorola MC146818A'''. Its oscillator is '''Y1, 32.768 kHz''', with '''C17 27 pF''' and '''R16 2 MΩ''' round a '''14069''' inverter chain at '''ZM28A/B/C'''. The standby network is a diode chain '''CR1, CR2, CR4, CR6''' from +5 V with '''R10 620 Ω''' fed from +12 V, '''R12 1 kΩ''' to CKFS, '''R9 10 kΩ''', and '''C16 0.0056 µF''' on RESET. '''ZM50''' is a 74ALS244 buffering the floppy status lines onto XD0–XD7. That independently confirms what [[IBM PS/2 Model 50]] states from IBM's own documentation: the Model 50's real-time clock is an '''MC146818A with a separate lithium cell''', not a Dallas potted module. It is also the exception to the rule that no component values were ever published for these machines. <pdf> File:IBM PS2 Model 50 Schematics.pdf </pdf> Download it here: [[:File:IBM PS2 Model 50 Schematics.pdf|IBM PS2 Model 50 Schematics.pdf]]
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