Editing Acorn BBC Microcomputer (section)

==Overview==
Unusually for an 8-bit home computer, these computers used discrete mechanical switches, mounted in aluminium and steel plates. Switches were sourced from [[Futaba]], [[PED]] and [[SMK]]. The three different brands of switch used different [[keycap mount]]s and the keycaps are not interchangeable between keyboard types, except presumably the two SMK types.

All the variants were supplied with [[spherical keycap|spherical]] [[double-shot molding|double-shot]] keycaps in a curious combination of black (technically very dark brown), flame red for the [[function keys]], and olive drab for the [[cursor keys]]. The differences between the variants are minor.

The keyboard is integrated into the computer; it is secured by two screws and nuts and is extremely easy to remove, after removing the four screws that retain the case lid. Unusually for a plate-mounted keyboard, it is the PCB and not the mounting plate that is screwed down. The keyboard has two cables that connect to the motherboard: one ribbon cable for the matrix, and the speaker cable, as the speaker is integrated into the keyboard. The keyboard also has capacity for fitting a ROM socket, usually used for speech synthesiser ROMs, and a [[DIP switch]] bank for setting hardware configuration.

<gallery widths="250" heights="187">
File:Straight outta Comptec.jpg|Type 3, 4 and 1 keycaps and switches; these types all used Comptec keycaps
</gallery>

===Operation===
Unlike a protocol such as the [[AT keyboard interface]], the BBC Micro keyboard does not transmit keypresses as they occur. All the matrix rows are connected at one end to a 74LS30 NAND gate,<ref name="AUG-489-90" /> which is in turn connected to CA2 (port A control line 2) of the System VIA; this control line generates the keyboard interrupt. (The VIA chips are MOS 6522 Versatile Interface Adapter ICs found on the computer motherboard.) The matrix columns are strobed continually at 1 MHz; when any key is pressed, the OS receives the interrupt signal, and takes control of the keyboard scanning. During controlled scanning, the matrix rows are sensed individually to identify which keys are pressed.<ref name="Arduino" /> Detection of key ''release'' however is unexplained.

Acorn MOS supports two-key rollover; two locations in zero page (&EC and &ED) hold the non-modifier keys currently depressed, or zero otherwise.<ref name="AUG-142" />  Row zero of the matrix has anti-ghosting diodes fitted<ref name="AUG-489-90" /> which allows contacts in this row to be detected simultaneously with any two keys elsewhere in the matrix.  Row zero contains the modifier keys '''shift''' and '''control''' and the start-up options.

===Start up options===
The BBC Micro has no writable, non-volatile memory. To partially alleviate this, eight bits of "start up options" can be set in hardware. These options control the default screen mode (bits 0–2, switches 6–8), reversal of shift+break (bit 3, switch 5), and diskette drive parameters (bits 4 and 5, switches 3 and 4); bit 6, switch 2, is not used.<ref name="AUG-489-90" /><ref name="8bs" />  In a machine fitted with the DNFS filing system ROM bit 7, switch 1, is used to control the default filing system should both disc and network hardware be fitted, otherwise it is not used.<ref name="DNFS" /> These eight bits occupy columns two through nine of the first row of the keyboard matrix and function as keys.

Solder pads are provided on the keyboard PCB to affix a DIP switch block; alternatively the settings can be hard-wired using links. By default, each of these eight bits defaults to zero.