Keyboard

The keyboard is attached to the slow bus and accessed by the system VIA. There are 74 keys including BREAK. 73 keys and 8 keyboard links are wired in a 10 × 8 matrix; the two SHIFT keys are connected to one cell and cannot be told apart. The BREAK key is on a separate circuit and resets the computer.

All keys except CTRL and SHIFT cause an interrupt when pressed, which makes the BBC scan the keyboard to identify the key(s) that are held down.

The keyboard has an unusual bit-paired layout, unlike the typewriter-paired layout that is standard on British PCs. Pressing SHIFT or CTRL neatly adds or subtracts a power of two from the ASCII value of the base character, except for the singleton characters space, 0, @, _, £ and DELETE. The layout is shown in Chapter 1 of the User Guide.

There is a circuit diagram of the keyboard on page 489 of The Advanced User Guide.

BBC Micro keyboard types

8bs.com Type	Seen on	Key legends	Key sound	PCB colour	Chassis plate	Key switches	Space bar support	DIP switch orientation
Type 1	B, ?	Large	Flat	Beige	Narrow margins, lip at front	White, round body, square-edged shoulder, cross head	Plastic tabs and wire rod (concealed)	East-west (under DELETE)
Type 2	B, B+	Small	Echoing	Tawny brown	Wide margins, deep skirt	Black, square body, chamfered shoulder, cross head	Wire rod support	North-south
Type 3	?	Large	Flat	Tawny brown	Wide margins, shallow skirt	Dark grey, deep bevelled shoulder, square cup head	Plastic guides?	North-south
Type 4	?	Large	Echoing	Beige	Narrow margins, lip at front	Black, square body, chamfered shoulder, cross head	Wire rod support	East-west (between SHIFT and DELETE)

-- beardo 00:01, 18 October 2007 (BST)