Acorn System 2 Troubleshooting Guide
A working Acorn System 2 shows "Acorn Cos" and an asterisk prompt on the display when the reset key is pressed.[1] Fault-finding starts from that sign-on. The CPU and cassette cards are Acorn System 1 boards, so the Acorn System 1 Troubleshooting Guide covers their circuits. The System 2 sets them up differently, as below.
No sign-on
[edit | edit source]- Supply: 5 V at the backplane, at least 1.5 A (2 A with 8 KB of RAM). No card has its own regulator.[1]
- Reset: the reset button on the keyboard connects to 0 V on pin 19 of the CPU card's keyboard connector.[1]
- COS ROM: the Cassette Operating System is in IC7 on the CPU card. It needs a track broken on side 2 and a wire link from pin 5 to pin 9 of the ADR SEL position; check both on a card that has been changed or swapped from a System 1.[1]
- Clock and processor: check the 1 MHz clock and the 6502 as on the System 1 card.
- Display: if the processor is running but nothing appears, check the VDU card (below).
To narrow a fault down, run the system with the CPU, VDU and memory cards only, then add the cassette card.
Display faults
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| No picture | Video lead, monitor or VDU card | Check the composite output on pin 15 of the VDU card's front connector (0 V on pin 14), 1 V into 75 Ω[1] |
| Unstable characters, or wrong width | Preset R3 on the VDU card | Adjust R3 with the COS running[1] |
| Colours shown as greys | Normal on composite | Colour needs the card's RGB and sync outputs and a TTL RGB monitor or colour encoder[1] |
| Wrong or corrupt characters | SAA5050, the two 2114-3 RAMs or the MC6845 | Reseat and substitute[1] |
Keyboard
[edit | edit source]The ASCII keyboard connects to port B of the 8154 (IC2) on the CPU card. It needs 7-bit data, +5 V on pin 20, 0 V on pin 1 and a strobe that goes low while a key is pressed. On the optional 20-way header the pins are different; match the pin 1 arrows on header and socket.[1] COS commands must be typed in capitals.[1]
Cassette
[edit | edit source]COS messages during a tape load are "Play drive 0", "Rewind drive 0" if the tape was started part way through the file, and "sum" if a block fails its checksum. After a checksum error, rewind to before the failed block and use FLOAD to continue loading.[1]
If tapes will not load at all:
- Check the four backplane links between the CPU and cassette cards: A29 to B11 (φ2), B19 to B12 (CASIN), B20 to B13 (CASOUT), B21 to B14 (NHITON).[1]
- Check the cassette card's link from IC6 pin 3 to B14.[1]
- Set the playback level so that IC7 pin 7 on the cassette card shows more than 2 V peak to peak without clipping (R12 sets the input gain, R3 the output).[1]
BASIC
[edit | edit source]The BASIC command starts the interpreter; if BASIC is missing, the system crashes and must be reset.[1] On the memory card, the BASIC ROM is in IC17 with a link at ROM 6; issue 1 cards hold BASIC in two EPROMs, IC17 (low) and IC18 (high). The 4 KB of user RAM in IC3–IC10 needs the link at RAM 1.[1]
COS error messages
[edit | edit source]| Message | Meaning |
|---|---|
| Com? | Command not recognised |
| Syn? | Command recognised but syntax wrong |
| sum | Checksum failed at the end of a tape block |
Source: Acorn's System 2 Technical Manual, chapter 6.[1]
Related pages
[edit | edit source]- Acorn System 2
- Acorn System 2 Maintenance Guide
- Acorn System 2 Capacitor Replacement Guide
- Acorn System 1 Troubleshooting Guide
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 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). Parts lists pp. 2-1 to 2-4; CPU card set-up and COS ROM link pp. 3-6 to 3-8; memory card p. 3-10; backplane links p. 3-12; power supply and keyboard pp. 5-1 and 5-2; cassette and display pp. 5-2 to 5-4; Cassette Operating System commands and messages, chapter 6.