The PCjr: an 8088 (16-bit core, 8-bit bus) with a wireless keyboard and a custom video gate array.
| Chip | Where | Role |
|---|
| 8088 microprocessor | ZM40 | CPU with a 16-bit core and an 8-bit external bus. Used in the IBM 5150, 5160, 5155, PCjr and the HERO 2000. |
| 8284A clock generator and driver | ZM8 | Crystal oscillator, divide-by-3 CPU clock, ready synchronizer and reset for the 8088. |
| 8259A programmable interrupt controller | ZM32 | Eight interrupt inputs (IRQ0 to IRQ7), priority logic and the vector for the 8088. |
| 8253-5 programmable interval timer | ZM23 | Three 16-bit counters: time-of-day tick, DRAM refresh pacing and the speaker tone. |
| 8255A-5 programmable peripheral interface | ZM22 | Three 8-bit programmable I/O ports (A, B, C). |
| MC6845 CRT controller | ZM21 | Makes sync, display enable, cursor and the video RAM refresh addresses. |
| PCjr video gate array | ZM36 (PCjr Technical Reference, sheet 4 of 17, and the layout on sheet 17); IBM 1503730 on the photographed board | PCjr video: modes, palette and the video memory interface. |
| SN76496 sound generator | ZM20 | Three tone voices and a noise voice. The PCjr sound chip. |
| INS8250 asynchronous communications element (UART) | ZM9 (PCjr Technical Reference, sheet 8 of 17); INS8250N on the photographed board | Serial port: one UART with a built-in baud rate generator and modem control lines. |
| 4164 DRAM 64K×1 | ZM61, ZM62, ZM68, ZM69, ZM71, ZM72, ZM76, ZM77 (PCjr Technical Reference, sheet 4 of 17); AMK4164ANP-15 on the photographed board | Main RAM (8 chips = 64 KB) |
| 23256 mask ROM 32K x 8 | XX1 and ZM63, 32K x 8 each (PCjr Technical Reference, sheet 2 of 17, draws two MK38000). IBM 1504036 (MK38050N-25) at XX1 and 1504037 (TMM23256P) at ZM63 on the photographed board | 32K x 8 mask ROM in the 27256 pinout. |
| 2732-pinout 4K x 8 ROM | ZM64, character ROM (IBM 1503723 on the photographed board). PCjr Technical Reference, sheet 5 of 17, draws an MK34000. | Character ROM in the 2732 pinout: IIe and IIc video ROM (4K x 8); PCjr character ROM (2K x 8 fitted, socket wired for a 2716 or 2732) |
| 24S10 / 82S129 bipolar PROM 256 x 4 | ZM56, memory decode: -VIDEO RAM CS, -ROM SEL and -MEM ON CARD (PCjr Technical Reference, sheet 5 of 17); TBP24S10N on the photographed board | Memory address decoder: a 256 x 4 lookup table that turns high address bits into chip selects. |
| 28L22 bipolar PROM 256 x 8 (PCjr I/O decoder) | ZM52 (PCjr Technical Reference, sheet 6 of 17); TBP28L22N on the photographed board | I/O address decoder: turns XA3-XA9 and XIO/-M into the chip selects of the planar I/O devices. |
| Rail | Expect | Measure at |
|---|
| +5 V | 4.75 to 5.25 V (5 percent) | jr-psu pins 4, 5, 6, 7 (power board connector); 8088 pin 40 |
| +12 V | 11.4 to 12.6 V (5 percent) | jr-psu pin 1 and pin 10 (power board connector) |
| -6 V | -5.04 to -6.96 V (16 percent) | Not on the power board connector table |
| Transformer AC | 15.0 to 30.0 V AC (IBM HMS p. 3-020-8) | jr-power pin 1 to pin 3, measured on the transformer cable plug unplugged from connector P, system switched off |