Editing BBC Micro analogue in

The '''Analogue in''' port is a D-subminiature 15-pin socket on the back of the [[Acorn BBC Microcomputer]]. The port provides four analogue channels, in two pairs, with each pair having an analogue ground and reference voltage. In addition, two pairs of pins allow for detecting two pushbuttons. A light pen strobe allows the port to be used for light pens.

==Wiring and usage==

The pins are as follows:

{| class="wikitable"
! Port !! Assignment
|-
| 1 || +5 V
|-
| 2 || 0 V
|-
| 3 || 0 V
|-
| 4 || Analogue channel 3
|-
| 5 || Analogue ground
|-
| 6 || 0 V
|-
| 7 || Analogue channel 1
|-
| 8 || Analogue ground
|-
| 9 || Light pen strobe
|-
| 10 || Pushbutton 1
|-
| 11 || Joystick 2 reference voltage (1.8 V)
|-
| 12 || Analogue channel 2
|-
| 13 || Pushbutton 0
|-
| 14 || Joystick 1 reference voltage (1.8 V)
|-
| 15 || Analogue channel 0
|}

The provision of a single socket means that pairs of joysticks must share a single plug. In theory, a splitter cable could also be produced should two people have already purchased separate joysticks.

The joystick wiring, for a pair of joysticks, is as follows:

[[File:BBC Micro analogue in (two joysticks).svg|centre]]

==Analogue to digital conversion==
In the Advanced User Guide for the BBC Microcomputer (third edition, November 1983), the analogue input is described as 10 bit, with full reference voltage being reported as 1023. A hardware register is available to select between 8-bit sampling and 10-bit sampling; selecting 8-bit sampling decreases the ADC delay from 10 ms to 4 ms. The decoder chip is a 7002.

BBC Microcomputer System User Guide, 1982, indicates that the ADC produces figures in the range 0 to 4095, and for the purposes of the API these are scaled up to 0 to 65520 to allow for future ICs to provide more accurate readings. The reference voltage is 1.8 V, with this giving 65520. The 1984 edition of the manual, for the Model B+, indicates that 1 V gives around 35158, and that while the decoder chip is 12-bit, only 10 bits of accuracy should be used unless great care is taken.

[[Category:Game controller protocols and interfaces]]