Acorn System 2
 | |
| Acorn System 2 in its 19-inch card frame (Control Universal catalogue image, via Chris's Acorns) | |
| Specifications | |
|---|---|
| Manufacturer | Acorn Computers Ltd |
| Type | Eurocard microcomputer, kit or assembled |
| Released | 1980 |
| Intro price | £320, or £480 with power supply; keyboard £136 (1982) |
| CPU | 6502 at 1 MHz |
| Memory | 4 KB RAM (expandable to 8 KB on the memory card); 2 KB COS ROM; 4 KB BASIC ROM |
| Storage | Cassette, 300 bit/s CUTS |
| Display | 40 × 25 teletext (MC6845 and SAA5050) |
| Sound | — |
| OS / Firmware | Cassette Operating System (COS) and Acorn BASIC |
| Predecessor | Acorn System 1 |
| Successor | Acorn System 3 |
The Acorn System 2 is a 6502 microcomputer built from Acorn's Eurocards in a 19-inch card frame. Acorn's technical manual for it is dated January 1980. It was sold as a kit or assembled, and the buyer chose the ASCII keyboard, display, cassette recorder and power supply.[1] In 1982 the System 2 cost £320, or £480 with a power supply, plus £136 for the keyboard.[2]
Cards
[edit | edit source]The basic System 2 has four Eurocards on an eight-slot backplane (200,011):[1]
| Card | Board | Contents |
|---|---|---|
| 6502 CPU | 200,000, issue 3 or later | 6502, INS8154 (IC2) for the keyboard, two 2114 RAMs, the 2 KB Cassette Operating System ROM in IC7, address decoding. The System 1 monitor PROMs (IC5, IC6) and the regulator (IC13) are not fitted. |
| Cassette interface | 200,001, issue 2 or later | The System 1 keyboard card without its display, 7445 and keypad |
| VDU interface | 200,002 | MC6845 CRT controller, SAA5050 teletext character generator, two 2114-3 RAMs; 40 × 25 characters |
| Memory | 200,003, issue 2 | 4 KB of 2114 RAM (IC3–IC10) with sockets for 4 KB more; BASIC ROM in IC17 and a socket for a second ROM in IC18 |
The CPU and cassette cards are the Acorn System 1 boards set up differently.[1] The left-hand backplane slot has its address bus isolated for bus buffers, so the four cards go in the next four slots.[1] A 6809 processor card was available instead of the 6502.[2][3]
Power
[edit | edit source]The System 2 needs a single stabilised 5 V supply of at least 1.5 A (2 A with 8 KB of RAM), wired to the backplane, or a 5 V 3 A module in the 2.8-inch space at the left of the frame. If the supply's 0 V is not earthed, the card frame must be earthed through the backplane's earth wire. None of the four cards has its own regulator.[1]
Display and cassette
[edit | edit source]The VDU card gives a 1 V, 75 Ω composite signal on pin 15 of its front connector (0 V on pin 14). On composite the colours appear as shades of grey; colour needs the card's RGB and sync signals and a TTL RGB monitor or a colour encoder. After switching on, preset R3 on the VDU card is set for stable characters of the right width.[1] The cassette interface is the System 1 CUTS design at 300 bits per second.[1]
When the system is working, pressing the reset key shows "Acorn Cos" and the COS prompt, an asterisk.[1]
Maintenance and repair
[edit | edit source]- Acorn System 2 Maintenance Guide
- Acorn System 2 Troubleshooting Guide
- Acorn System 2 Capacitor Replacement Guide
Documentation
[edit | edit source]- Acorn System 2 Technical Manual: Acorn's System 2 Technical Manual (issue 1, January 1980), chapters 1 to 8, in Ben Harris's 1998 text transcription (PDF).
- Acorn System 1 Technical Manual and Acorn System 1 Circuit Diagrams: the CPU and cassette cards.
- Acorn System Catalogue: the Eurocard range.
Related pages
[edit | edit source]References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Acorn Computers, System 2 Technical Manual, issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as Acorn System 2 Technical Manual (PDF). Chapter 1 introduction; chapter 2 parts lists (pp. 2-1 to 2-4); chapter 3 electrical assembly (CPU card p. 3-6, cassette interface p. 3-8, memory card p. 3-10, backplane pp. 3-11 and 3-12); chapter 4 card frame (p. 4-2); chapter 5 power supply, keyboard, cassette and display (pp. 5-1 to 5-4); chapter 6 Cassette Operating System.
- ↑ 2.0 2.1 "Acorn System", Wikipedia (retrieved 2026-10-01). System 2 configuration, 6809 option, 1982 prices (£320, £480 with power supply, keyboard £136).
- ↑ Whytehead, Chris. "Acorn System 2", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). 19-inch card frame with an 8-slot backplane; 1 MHz 6502 CPU with 2K COS, 40 × 25 8-colour VDU, memory board with 4K RAM and 4K BASIC ROM, cassette interface; links to the System 2 Technical Manual text on Ben Harris's site.