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IBM PS2 Model 50 Schematics

From RetroTechCollection

The PS/2 Model 50 schematic set is sixteen pages of circuit diagrams for the PS/2 Model 50 (8550) planar, Revision 4, dated 21 May to 2 June 1991.

Sheet 1 is missing. Page 1 of the PDF is a placeholder reading "sheet 1 Missing from Original Set"; the scan runs from sheet 2 to sheet 16. Whatever sheet 1 covered is not here and was already absent when this set was scanned.

These sheets are not an IBM publication. Every title block carries the name Maintenance Information Technologies and the words "COPYRIGHT APPLIED FOR", together with this notice:

"Servicing of any electronic product should only be performed by qualified personnel. This drawing and information has been Copyrighted by the company listed. Its content has been sold for the unlimited use to you. The only restrictions are that it not be copied for republication or distribution outside your company. No patent liability is assumed with respect to the use of information contained herein. Maintenance Information Technologies believes the information herein to be correct. We are not responsible for any typographical errors; no direct or implied warranty is made to its accuracy. Specifications subject to change without notice."

Maintenance Information Technologies was a third-party publisher of service schematics for the independent repair trade in the late 1980s and early 1990s. IBM itself never published a PS/2 schematic.

What it contains

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Fifteen sheets, numbered 2 to 16, each titled "PS/2 50":

  1. (sheet 1 missing from the original set)
  2. CPU
  3. MEMORY
  4. CLOCKS
  5. BUS CONNECTIONS
  6. SERIAL I/O
  7. INTERRUPTS
  8. KEYBOARD I/F
  9. PRINTER I/O
  10. DISK I/F
  11. REAL TIME CLOCK
  12. DMA I/F
  13. VIDEO I/F
  14. BUS CONNECTORS/B
  15. I/O CONTROL
  16. BUS CONTROLLER

The drawings carry designators and values. Sheet 13 (Video I/F), for instance, shows R68 33 Ω and C175 68 pF on the video output network.

What it does not contain

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No text layer. These are scanned drawings. `pdftotext` returns almost nothing and OCR returns garbled fragments, so read them as page images, not as text.

No parts list, no bill of materials, no board silkscreen map and no power distribution sheet. The component values that are shown are the ones drawn on the signal circuits themselves. Bulk electrolytics and the planar decoupling do not appear, so these sheets will not give you a recapping list — but they will give you the value of a specific resistor or small capacitor in a circuit you are tracing, which is more than any IBM publication does.

There is no revision history and no statement of which planar revision a Model 30 or Model 50 sheet was drawn from.

What it settles

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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.

Download it here: IBM PS2 Model 50 Schematics.pdf

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