Acorn 6502 Second Processor Documentation

Acorn's 6502 Second Processor documentation hosted here comprises the service manual, the user guide and the main board circuit diagram for the Acorn 6502 Second Processor.

Service manual

The 6502 Second Processor Service Manual is Acorn Computers' dealer service manual, part number 0408,003, issue 1, May 1984. The hosted PDF has 45 pages. Its text was retyped from the original and keeps a number of OCR errors: "1C" for "IC" throughout, "LK5 to LK5" for the link range in section 5.4, a first link paragraph headed "LK2" that describes LK1, and "IC13" for the Tube chip in section 5.3, which is IC1. The drawings at the back are scanned images.

Contents

Service manual contents (printed pages)

Section	Page
1 Introduction	1
2 Packaging and installation (DNFS and HIBASIC ROMs, T315mA fuse at the rear)	2
3 Specification: processor, Tube operating system, BASIC, power supply, environmental, dimensions	3 to 4
4 Disassembly and assembly	5
5 Circuit description: power-up, boot mode, after power-up, PCB links, Tube connector pinout	6 to 8
6 The Tube: registers, pinout, timing	9 to 14
7 Fault finding: start-up message printed, total failure, power supply, oscillator, boot mode checks, run mode checks, DRAMs, CPU, Tube IC1	15 to 21
Appendix: IC numbers and functions, general assembly, block diagram, main PCB circuit diagram, assembly drawing, power supply board circuit diagram, parts list	23 to 38

What it settles

• The power supply specification: 216 to 264 V AC, 47 to 63 Hz, 14 W, 1 A maximum at +5 V (section 3.4).
• The power supply test: disconnect the brown (+5 V) and black (0 V) leads from the main board, load them with 10 ohms at 2.5 W, and expect 4.95 to 5.25 V with no more than 50 mV of noise; set the trimmer on the power supply board for 5 V. The main board draws 800 to 1000 mA (section 7.3.1).
• The over-voltage crowbar: fuse FS1 and thyristor TH1 on the main board, with the thyristor blowing the fuse on over-voltage (section 7.3.1).
• The power supply board circuit (p. 35): mains transformer T1, Schottky rectifiers D1 and D2 (11DQ04), reservoir C1 4700 µF 16 V, pass transistor TR1 (TIP42), output capacitor C5 47 µF 35 V, trimmer VR1 (100 ohms, "set 5V") and output fuse F1 (1 A).
• The main board parts list (p. 37): the only polarised capacitor on the main board is C3, 10 µF 10 V tantalum; FS1 is 2 A, 20 × 5 mm.
• The diagnostic sequence by signal: 12 MHz at IC8 pin 12, boot-mode PHI IN of 187.5 kHz at IC2 pin 37 for 0.25 s after BREAK, reset at IC2 pin 40, SYNC at IC2 pin 7 and the 68 kHz refresh request at IC13 pin 15 (sections 7.3.2 to 7.5.2).

What it does not contain

• The manual has two different figures for the rear fuse: section 2 says T315mA, while the case parts list (p. 38) gives a 20 × 5 mm 1 A fuse.
• It says there should be 5 V on Tube ULA pins 2 and 3. Pin 3 is fed through R1 (12 ohms, 1 W) on the circuit diagram, and owners measure about 3.1 V there on working units.
• No component values for the rest of the main board are printed in the text; the circuit diagram in the scan is small. A clearer copy of the main board drawing is hosted separately (below).

User guide

The 6502 Second Processor User Guide is Acorn's owner's manual, part number 408000, issue 2, March 1984. The hosted scan has 58 pages and a text layer.

User guide contents (printed pages)

Part	Chapters	Pages
Part 1, 6502 Second Processor	Introduction; installation (DNFS and Hi-BASIC ROMs, connecting the unit); starting up; system memory (I/O processor and second processor memory maps); using standard BASIC; Hi-BASIC; expanding the memory; copying ROMs; distinguishing between memories; using the second processor from assembly language	5 to 24
Part 2, Hi-BASIC (BASIC II)	Changes to keywords; error handling; precision; new commands; new assembler features	29 to 43
Appendices	A: fitting the Hi-BASIC and DNFS ROMs; B: disc and Econet filing system differences	45 to 48

It covers installation and use only. There is no circuit information and no fault finding beyond "contact your nearest Acorn Service Centre" if the start-up message does not appear (p. 9).

Main board circuit diagram

Acorn's drawing A1/108,000/C, "6502 2nd. Processor", issue 4 (July 1984), in two halves. These are conversions of Ben Harris's 600 dpi scans and are much easier to read than the copy in the service manual.

• 
  Left half: Tube connector, Tube ULA IC1, 6502C IC2, boot ROM IC3, reset IC26
• 
  Right half: oscillator, clock and refresh counters, crowbar, DRAMs IC18 to IC25

Pages that cite these documents

• Acorn 6502 Second Processor

Download: Acorn 6502 Second Processor Service Manual.pdf

Download: Acorn 6502 Second Processor User Guide.pdf