I’m working on a project involving the keyboard from a BBC Master Compact. I couldn’t find an accurate pinout on the web anywhere so I got my multimeter out and mapped it.
LED Power 01 - 02 Column Row 03 - 04 Break Row 05 - 06 Row Column 07 - 08 Row Column 09 - 10 Column Column 11 - 12 Column Column 13 - 14 Column Column 15 - 16 Column Column 17 - 18 Column Row 19 - 20 SHIFT+CTRL Row 21 - 22 Column LED Caps 23 - 24 Column LED Anode 25 - 26 LED Shift Lock
The LEDs all have suitable resistors on them and can run directly from 5V. Break is pulled up by the same line as the power LED uses for 5V so is effectively on it’s own row. Inside the BBC it is used to trigger a hardware interrupt IIRC.
The SHIFT+CTRL pin (20) is special. These two keys use diodes that allow them to be read by pulling lines 21 (SHIFT) and 24 (CTRL) low in turn.
EDIT: Made some minor corrections