Acorn Electron Plus 1
| Acorn Electron Plus 1 | |
|---|---|
 |
|
| An Electron with the Plus 1 fitted at the back (photo: Bilby, CC BY 3.0) | |
| Manufacturer | Acorn Computers; later Advanced Computer Products under licence |
| Type | Expansion unit |
| Release date | 1984[1] |
| Price | £59.90[2] (£59 in Acorn User's review[1]) |
| Interface | Electron expansion connector |
| Compatible | Acorn Electron |
| Model | ALA11 |
| Connectivity | Two ROM cartridge slots, Centronics printer port (26-way), analogue port (15-way D) |
| Power | From the Electron |
The Acorn Electron Plus 1 is the first expansion unit Acorn made for the Acorn Electron. It screws onto the Electron's rear expansion connector and adds two ROM cartridge slots, a Centronics parallel printer port and a four-channel 8-bit analogue interface for joysticks, games paddles or voltage measurement. A ROM in the unit holds the printer, analogue and cartridge software.[1][3] Acorn's sales code was ALA11.[4]
Acorn User reviewed it in July 1984 at £59; Wikipedia gives £59.90.[1][2] Acorn stopped making it in November 1985, and Advanced Computer Products restarted production under licence in 1987 using Acorn's moulds.[2] PRES later published its own Plus 1 user guide (1988).[4]
Hardware
[edit | edit source]
The Plus 1 has no power supply of its own and runs from the Electron's.[3] The board carries an ADC0844 analogue-to-digital converter and the Plus 1 ROM, both socketed; Acorn User found every other chip soldered to the board.[1] Chris Whytehead's board is marked 0219,100 issue 2.[4] Of the 8 KB utility ROM, only 3 KB is used, which left room for replacement ROMs such as the PRES AP2 and the Slogger Expansion ROM.[2][4]
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | +5 V | 9 | Not connected |
| 2 | 0 V | 10 | PB1 (fire button 1) |
| 3 | 0 V | 11 | VREF (1.8 V) |
| 4 | CH3 | 12 | CH2 |
| 5 | Analogue ground | 13 | PB0 (fire button 0) |
| 6 | 0 V | 14 | VREF (1.8 V) |
| 7 | CH1 | 15 | CH0 |
| 8 | Analogue ground |
The analogue inputs accept 0 to 1.8 V. A device that uses the reference must present at least 10 kΩ; a device that generates its own voltage must drive at least 1 mA. Each conversion takes 10 ms per active channel, so four channels refresh every 40 ms.[3] Acorn's own ANH01 joysticks suit the port.[3]
The printer socket is a 26-way header. Acorn's cable ANG04 maps it to a 36-way Centronics plug: strobe on pin 1, data 0 to 7 on pins 3 to 17 (odd), acknowledge on pin 19 and ground on the even pins 2 to 24.[3] The printer driver drops the line feed after each carriage return unless *FX6,0 is set.[3]
Fitting
[edit | edit source]- Switch off and unplug the Electron, and disconnect the other leads. Never fit or remove the Plus 1 with the power on.
- Turn the Electron upside down with the expansion connector towards you and remove the plastic cover from the connector.
- Check that the threaded ends of the two large screws on the Plus 1 are flush with its base. Turn the Plus 1 upside down, push the Electron's connector into the Plus 1 socket, and screw the two screws into the Electron with a coin. Do not overtighten. If the screws will not engage, reseat the connector.
- Turn the pair over, reconnect the power and check that the Electron still runs a known program.[3]
Acorn warns that repeatedly disconnecting and reconnecting the Plus 1 wears the expansion connector contacts.[3] With a Plus 3, the Plus 1 fits to the back of the Plus 3 instead of the Electron.[5]
Cartridges
[edit | edit source]Cartridges plug into the two shuttered slots on top, label towards the keyboard, right-hand end first, and click home. ROM-filing-system cartridges (games and applications) boot when BREAK is pressed. Language cartridges page in at &8000 to &C000 in place of BASIC; with two languages fitted, the one nearer the keyboard takes priority, and *BASIC returns to BASIC.[3] The cartridge slots carry extra control lines beyond those on the Electron's own edge connector, and other makers later used them for disc interfaces, second processors and sound cartridges.[2]
Faults and problems
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Electron dead or misbehaving with the Plus 1 fitted | Dirty or worn edge connector; a faulty peripheral | Remove the Plus 1 and check the Electron alone. Clean the Electron's edge connector fingers with a non-abrasive eraser and a soft cloth, refit, then reconnect each peripheral and cartridge in turn[3] |
| Tape commands not working after using a cartridge | Filing system still set to ROM | Type *TAPE[3]
|
| Tape loading errors, or a program that runs without the Plus 1 but not with it | Extra interrupt servicing and workspace in page &D | Use modes 4 to 6, turn off unused analogue channels with *FX16, or disable Plus 1 servicing with *FX163,128,1 (re-enable with *FX163,128,0)[3][2]
|
| Cannot get back to BASIC from a cartridge | Protected cartridge disables BREAK | Remove the cartridge, then press BREAK[3] |
| Printer prints every line over the last | The driver suppresses the line feed after each carriage return | Set *FX6,0 to send line feeds[3]
|
| Joystick or paddle readings wrong or noisy | Device impedance below 10 kΩ, or wiring | Check the device against the pinout and impedance figures above[3] |
The Plus 1 has no power supply circuitry, and Chris Whytehead's board photograph shows disc ceramic capacitors and no large electrolytics.[4] Acorn's own service advice centres on the contacts: clean the Electron's edge connector and avoid repeated reconnection.[3] For the Electron itself see the Acorn Electron Troubleshooting Guide.
Documentation
[edit | edit source]- The Electron Plus 1 User Guide (Acorn, 419100 issue 1, March 1984).[3]
- Acorn did not include the Plus 1 in the Acorn Electron Service Manual, which covers the Electron alone.
Related pages
[edit | edit source]- Acorn Electron
- Acorn Electron Plus 3
- Acorn Electron Maintenance Guide
- Acorn Electron Troubleshooting Guide
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 "Electron special: expansion factor", Acorn User, July 1984. Chris's Acorns copy (retrieved 2026-10-02). £59; adds 100 mm (4 in) to the depth of the computer; fixed by two large bolts turned with a coin; ADC0844 converter and an EPROM (version 1.00 on the review unit) socketed, all other chips soldered; the nearer cartridge has priority; six cartridges at launch.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 "Acorn Electron", Wikipedia (retrieved 2026-10-02), sections "Acorn Plus 1" (two cartridge slots, four-channel analogue interface, Centronics port, £59.90; page &D used as workspace and the commands to disable it; cartridge slots carry extra control lines) and "Further developments" (Acorn production ended in November 1985; Advanced Computer Products made it under licence from 1987; only 3 KB of the 8 KB utility ROM used; the PRES AP2 replacement ROM).
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 Acorn Computers, The Electron Plus 1 User Guide, part no. 419100, issue 1, March 1984. Hosted on this wiki: File:Acorn Electron Plus 1 User Guide.pdf. Introduction p. 1; installation pp. 2 to 3; printer, paddles, cartridges, tape and disconnecting pp. 3 to 4; analogue interface and 15-way pinout pp. 6 to 7; paddles and joysticks pp. 7 to 8; other analogue peripherals p. 8; ADVAL and conversion timing pp. 9 to 10; printer cable ANG04 and pinout p. 12; printer drivers pp. 13 to 15; cartridges pp. 16 to 17; dealing with problems pp. 18 to 19; *FX summary pp. 20 to 22.
- ↑ 4.0 4.1 4.2 4.3 4.4 Whytehead, Chris. "Acorn ALA11 Plus 1", Chris's Acorns, The Centre for Computing History (retrieved 2026-10-02). Two cartridge slots, parallel printer port and analogue socket with four 8-bit channels; base for disc drives, sideways ROMs and other upgrades through the cartridge slots; his unit fitted with the Slogger Electron Expansion ROM; photographs including the circuit board.
- ↑ Acorn Computers, The Electron Plus 3 User Guide, part no. 0441,000, issue 1, October 1984, pp. 4 to 5. Hosted on this wiki: File:Acorn Electron Plus 3 User Guide.pdf. Fitting a Plus 3 to an Electron that already has a Plus 1.