Acorn BBC Microcomputer

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BBC Microcomputer keyboard
Acorn BBC Microcomputer.jpg
Branding BBC
Manufacturer Wong's, SMK, Philips?
Keyswitches Futaba MR-6C series
SMK J-M0404 series
Philips keyswitch
Switch mount Plate mount
Keycaps Double-shot spherical
Interface Proprietary internal

The BBC Microcomputer System from Acorn Computers Ltd, commonly known as the BBC Micro or Beeb, was a series of 6502-based 8-bit microcomputers introduced in the United Kingdom in late 1981/early 1982.

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, Philips and SMK. The three different brands of switch used different keycap mounts and the keycaps are not interchangeable between keyboard types, except presumably the two SMK types.

All the variants were supplied with spherical 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 and a DIP switch bank for setting hardware configuration.

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,[1] 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.[2] Detection of key release however is unexplained.

Acorn MOS supports two-key rollover; two locations in zero page (&EC and &ED) hold the keys currently depressed, or zero otherwise.[3]

Start up options

The BBC Micro has no 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); the final two bits are not used.[4][1] 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.

Keyboard types

Prior to the identification of the switches, the keyboards were classified by Chris Richardson of 8-Bit Software into four types,[5] which are not numbered in the order that they were introduced. These type numbers are used here for convenience.

Type 1

This is the original and most common keyboard type, with an aluminium plate and Futaba MR-6C series switches. This appears to have been manufactured from the 1981 to 1984. The OEM was Wong's. The keycaps were manufactured by Comptec; this is not only evident from the design, but Signature Plastics have confirmed that Wong's was a major customer.[6] "Keytop set 'Comtec'" [sic] (£20.83) and "Spacebar (Comtec)" (£0.66) are listed in a catalogue, but the specific keyboard for which it was intended is not given.[7]

Type 2

SMK J-M0404 series switches with angled keystems, mounted onto a steel plate. Known manufacturing dates are from 1984. The OEM was SMK, part number 401KBM-006-90RⒶ. The shift keys, space bar and possibly also tab use the brown and beige low-friction version of the switch.[5] These switches have been reported to be unreliable.[5] The keycaps were sourced from SMK, as they fit SMK's moulding style.

The return key uses a pair of low-force switches in tandem, connected together on the PCB.[8] These switches have the brown slider indicative of low friction switches, together with a beige base.

Type 3

Philips keyswitches[7] The mounting plate is aluminium. Also known to be manufactured in 1984. The PCB is not branded. The keycaps were supplied by Comptec.

Type 4

SMK J-M0404 series switches with upright keystems, on an aluminium plate. Also known to be manufactured in 1984. The OEM was Wong's. The keycaps were supplied by Comptec.

Later computers

The Acorn Electron and the original Master Series machines used Futaba low-profile linear switches. The Master Compact used conductive rubber domes over a PCB, and later Master 128s used Cherry MY switches.[5] The Futaba low-profile linear switch did not have anything close to the reliability level of the simplified linear model.

External links

References

  1. 1.0 1.1 Bray, Andrew C; Dickens, Adrian C; Holmes, Mark A — The Advanced User Guide for the BBC Microcomputer — pp. 489–90: Appendix J — The keyboard circuit
  2. Arduino Forum — BBC Micro Keyboard
  3. Bray, Andrew C; Dickens, Adrian C; Holmes, Mark A — The Advanced User Guide for the BBC Microcomputer — p. 142: OSBYTE &78 (120) *FX 120
  4. Bray, Andrew C; Dickens, Adrian C; Holmes, Mark A — The Advanced User Guide for the BBC Microcomputer — p. 246: OSBYTE &FF (255) *FX 255
  5. 5.0 5.1 5.2 5.3 8-Bit Software — See Their Insides
  6. Correspondence with Signature Plastics, 2015-10-02
  7. 7.0 7.1 BBC Micro Mailing List — Subject: Interesting DRAM catalogue
  8. Correspondence with Andrew from CJE Micro's, 2016-10-06