5 pin CONNECTOR at the computer
The IBM PC/AT (Model 5170) front-panel LED and keylock connector is a 5-pin Berg strip designated J20 in IBM’s 1984 Technical Reference. It carries power to the chassis power LED on pin 1 and a keyboard inhibit signal on pin 4, allowing a physical keylock switch fitted to the case to disable keyboard input. Pin 2 on IBM’s original keyed connector is a physical blank (absent pin used for polarisation); generic AT-clone boards commonly populate pin 2 as an additional ground. The connector uses 0.1-inch (2.54 mm) pitch friction-lock Berg-strip housings standard to AT-era front-panel wiring.
| Pin | Name | Description |
|---|---|---|
| 1 | LED | LED Power |
| 2 | GND | Ground |
| 3 | GND | Ground |
| 4 | KS | Key Switch |
| 5 | GND | Ground |
Signals
- LED — Power LED anode drive; connects to the positive leg of the front-panel power indicator LED.
- GND — Ground return (pins 2, 3, and 5). Pin 2 is a physical keying blank on IBM’s original keyed Berg strip; AT-clone boards typically wire it as ground.
- KS — Keyboard inhibit (Key Switch); pulled low by the front-panel keylock switch to disable keyboard input to the system.
Notes
The connector is a 5-pin single-row Berg strip at 2.54 mm pitch. IBM’s original J20 connector is keyed with a missing pin at position 2 to prevent reversed insertion; many AT-clone implementations substitute a ground pin at position 2. The keylock function (pin 4) disables keyboard scanning in hardware; the feature was dropped on ATX-era motherboards.