BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

32. WEST selects A 13 input to pin 26 of ROMs IC52 and IC88.
EAST selects +5v input to pin 26 of ROMs IC52 and IC88.
- This enables 24 pin ROMs to be used in the 28 pin socket.
33. WEST selects A 13 input to pin 26 of ROMs IC100 and IC101.
EAST selects +5v input to pin 26 of ROMs IC100 and IC101.
- This enables 24 pin ROMs to be used in the 28 pin socket.
34-38. These are used to provide contact with the ROM decoder (IC20)
and the chip select lines of ROMs 52, 88, 100 and 101, in order to
allow the use of extra ROM sockets on an external PCB. (Implemented
from issue 4.)
39. CLOSED adds colour burst signal to the black and white video
signal to produce PAL encoded video on the BNC socket.
OPEN Black and white video on BNC socket. (Implemented from issue 4.)
34