28L22 bipolar PROM 256 x 8 (PCjr I/O decoder) pinout
DIP-20. I/O address decoder: turns XA3-XA9 and XIO/-M into the chip selects of the planar I/O devices.
A 256 x 8 bipolar PROM used as the PCjr I/O decoder. Its outputs make the selects for the NMI register, the 8253 timer, the serial port and sound, the game port, the 8255 and the modem and diskette cards. IBM programs it, so a replacement must be burned from a dump. A bad one kills one or more I/O devices at once.
Open the interactive 28L22 pinoutPins
| Pin | Name | Type | Signal |
|---|---|---|---|
| 1 | A0 | addr | Address input 0 (XA3). |
| 2 | A1 | addr | Address input 1 (XA4). |
| 3 | A2 | addr | Address input 2 (XA5). |
| 4 | A3 | addr | Address input 3 (XA6). |
| 5 | A4 | addr | Address input 4 (XA7). |
| 6 | D0 | ctrl | Output 0. |
| 7 | D1 | ctrl | Output 1. |
| 8 | D2 | ctrl | Output 2. |
| 9 | D3 | ctrl | Output 3. |
| 10 | GND | ground | Ground. |
| 11 | D4 | ctrl | Output 4. |
| 12 | D5 | ctrl | Output 5. |
| 13 | D6 | ctrl | Output 6. |
| 14 | D7 | ctrl | Output 7. |
| 15 | /S1 | ctrl | Output enable 1, active LOW (grounded). |
| 16 | /S2 | ctrl | Output enable 2, active LOW (grounded). |
| 17 | A5 | addr | Address input 5 (XA8). |
| 18 | A6 | addr | Address input 6 (XA9). |
| 19 | A7 | addr | Address input 7 (XIO/-M). |
| 20 | VCC | power | +5 V. |
Fitted in
- IBM PCjr 4860 ZM52 (PCjr Technical Reference, sheet 6 of 17); TBP28L22N on the photographed board