Acorn 6502 Second Processor

Acorn 6502 Second Processor

The 6502 Second Processor in its "cheese wedge" case
Specifications
Manufacturer	Acorn Computers
Type	Tube second processor for the BBC Micro
Intro price	£199 at launch
CPU	6502C at 3 MHz
Memory	64 KB DRAM (eight 64 Kbit chips)
Storage	—
Display	—
Sound	—
ROM	Boot ROM (2732) holding 2 KB of the Tube operating system
Power	Internal linear supply, 216 to 264 V AC, 14 W
Dimensions	72 × 207 × 346 mm
OS / Firmware	—
Model no.	ANC01

The Acorn 6502 Second Processor is an external processor unit for the BBC Micro, connected through the Tube interface on the underside of the computer. It contains a 3 MHz 6502, 64 KB of RAM and its own mains power supply in a case shaped to sit to the right of the BBC Micro. With it connected, the BBC Micro becomes the input/output processor and the second processor runs the language and the user's program. Acorn's sales code for it was ANC01.

The unit came with two ROMs for the BBC Micro: DNFS, which holds the host half of the Tube software together with the disc and Econet filing systems, and Hi-BASIC, a version of BASIC relocated to give 44 KB for programs. Acorn's two documents give different speed claims: the user guide says programs run "up to 50% faster", the service manual says BASIC runs "up to twice as fast as the BBC Microcomputer alone".

Hardware

The processor runs from a 12 MHz crystal oscillator (X1 with IC8, a 74S04), divided by four to 3 MHz. The parts list calls for a 6502C (IC2), which the service manual describes as a "4MHz spec 6502"; the board photographed by Chris Whytehead carries a GTE 65SC02P-1 instead.

Main board ICs (service manual appendix and parts list)

IC	Part	Function
IC1	Tube ULA	Interface between the BBC Micro (host) and the second processor (parasite); socketed
IC2	6502C	Processor; socketed
IC3	2732	Boot ROM ("6502 BR") with the Tube operating system; socketed
IC4	74LS133	Decodes the Tube addresses
IC6	74LS74	Boot mode flip-flop and refresh timer reset
IC7	74LS393	Refresh row address counter
IC8	74S04	12 MHz oscillator and gating
IC11	74LS161 (74S161 in the parts list)	Divide by 16 in boot mode
IC12	74LS163	Divide by 4 for the processor clock
IC13	74LS163	Refresh timer
IC15 to IC17	81LS95	Three-state address buffers
IC18 to IC25	4164-15	64 Kbit DRAMs, 64 KB in total
IC26	74LS122	Power-up reset monostable

The remaining logic is 74LS00, 74LS02 and 74LS74/74S74 gating (IC5, IC9, IC10, IC14, IC27). Acorn's circuit diagram (drawing 108,000, issue 4, July 1984) names the DRAMs as 8264-15 and gives 74LS795 as an alternative for the 81LS95 buffers.

Main board, with the Tube ribbon cable at the right and the Ferranti Tube ULA beside it (photo: Chris Whytehead, CC BY-SA 3.0)

Boot mode

At power-on or BREAK the processor spends about a quarter of a second in boot mode. IC26 holds the processor in reset for about 100 µs, the clock is slowed to 187.5 kHz, reads come from the boot ROM and writes go to RAM, so the 2 KB of Tube code is copied to &F800 to &FFFF. When the software first touches a Tube address, IC4 and IC6 switch the ROM out and the clock rises to 3 MHz. From then on the processor runs entirely from RAM. The host half of the Tube code, 1 KB, is copied from the DNFS ROM into the BBC Micro at &400 to &7FF.

Links

The links are hard-wired for normal use. Section 5.4 of the hosted manual misnumbers the first of them; the circuit diagram shows it as LK1.

• LK1 powers the whole unit from the BBC Micro's supply. The second processor's own supply leads must be disconnected first, and Acorn advises against it because it may overload the BBC Micro's supply.
• LK2 connects HIRQ, the interrupt from the second processor to the host. Standard software does not use it, and with LK2 made the BBC Micro will not work while the second processor is switched off.
• LK3 selects where the host half of the Tube ULA gets its power. Acorn says it must not be altered.
• LK4 stands in for diode D1, which is not fitted because the disable feature was not implemented.
• LK5 sets the refresh rate for a 4 MHz processor and must not be changed at 3 MHz.

Power supply

Transformer and power supply board at the front, main board behind (photo: Chris Whytehead, CC BY-SA 3.0)

The supply is a linear regulator on its own board beside the mains transformer. Acorn rates it at 216 to 264 V AC, 47 to 63 Hz, 14 W, and 1 A maximum at +5 V. Its circuit (service manual p. 35) is a centre-tapped transformer T1 with Schottky rectifiers D1 and D2 (11DQ04), a 4700 µF 16 V reservoir C1, a TIP42 pass transistor TR1 driven by TR3 and TR4 (BC182) with a 3.9 V zener reference, a 100 ohm trimmer VR1 marked "set 5V", a 47 µF 35 V output capacitor C5 and a 1 A output fuse F1.

The main board has its own protection: a 2 A fuse FS1, and a thyristor TH1 triggered through a BZY88C 5.1 V zener (D2) that shorts the supply and blows FS1 if the voltage rises too far. A blown FS1 therefore means either a short on the board or a supply giving too high a voltage.

Section 2 of the service manual gives the rear panel fuse as T315mA, while the case parts list gives a 20 × 5 mm 1 A fuse. The moulded mains plug takes a 3 A fuse. Check what is fitted before replacing it.

Warning: the transformer primary and the rear fuse holder carry mains voltage. Unplug the unit before removing the lid.

Fitting and compatibility

1. Fit the DNFS ROM in one of the BBC Micro's sideways ROM sockets, replacing any separate DFS or NFS ROM, and fit Hi-BASIC if it is wanted. Without DNFS, or another filing system ROM containing the Tube host code, the second processor is not recognised.
2. With both units switched off, stand the second processor to the right of the BBC Micro and plug its ribbon cable into the socket marked Tube under the BBC Micro, matching the small ^ mark on the plug with the mark above the socket.
3. Switch on the second processor first and then the BBC Micro, or press CTRL BREAK after switching on. The screen should show "Acorn TUBE 6502 64K".
4. To use the BBC Micro on its own, switch the second processor off and press CTRL BREAK. The screen then shows "BBC Computer 32K".

On a BBC Master the Tube connector is configured as internal or external; with an external second processor, select the external Tube with *CONFIGURE EXTUBE (*CO. EXTUBE). Watford Electronics' DDFS 1.54T also contains the Tube host code.

Disassembly

1. Disconnect the unit from the mains.
2. Undo the three lid screws: two at the top of the back panel and one underneath, near the front, midway between the two rubber feet. Lift the lid off.
3. The transformer and power supply board are held by six screws. Acorn recommends leaving them in place unless necessary.
4. To remove the main board, pull off the two Faston tabs from the power supply (brown +5 V, black 0 V) and remove the four board screws.

Fault finding

Acorn's service manual notes that the board is hard to diagnose area by area, because only the 12 MHz oscillator works on its own: every other part needs the earlier parts to be working. Its procedure works forward from the oscillator.

Symptoms and checks (service manual section 7, with forum reports)

Symptom	Probable cause	Action
Blank screen with the cursor flashing at the top left	Most faults show this way	Work through the checks below, starting with the supply and the 12 MHz clock
"BBC Computer 32K" with the second processor switched on	Host does not see the second processor: no DNFS ROM, no power, faulty ribbon cable or failed Tube ULA	Check the ROM, the supply and the cable; try a known-good Tube ULA (IC1)
"Acorn TUBE 6502 64K" appears, then nothing works	Boot phase completed and the Tube is working; fault is in the DRAMs or the refresh circuit	Check refresh (IC13, IC7) and the DRAMs
Works on one BBC Micro but not another	Tarnished Tube connector or a worn ribbon cable; filing system ROM without Tube code	Reseat the plug repeatedly or clean the contacts; try another cable; check the ROM
Banner corrupted or printed in the wrong place; hangs at power-on but runs for hours once started	Intermittent: connections, the Tube ULA, or the power-on reset path	Replace the ribbon cable and reseat IC1 to IC3; owners have cured similar faults by replacing the Ferranti Tube ULA
Second processor fuse FS1 blown	Short on the board, or the supply giving too high a voltage and triggering TH1	Measure the supply off-load and into 10 ohms before fitting a new fuse

Acorn's checks in order

1. Check the T315mA rear fuse (see the note under Power supply), the leads and the mains switch.
2. Disconnect the brown and black leads from the main board and connect a 10 ohm 2.5 W resistor across them. Expect 4.95 to 5.25 V with no more than 50 mV of noise. If it is out of limits, set 5 V exactly with the trimmer through the hole in the power supply board; if that fails, replace the supply.
3. Reconnect and measure the current drawn by the main board: 800 to 1000 mA. Zero means an open circuit (check FS1, connectors and tracks). Too high with the voltage in limits means a shorting component; switch off and feel for the hot one.
4. Check the Tube ULA supplies. With only the BBC Micro switched on, IC1 pin 4 should be at 5 V against pins 1 and 5. With only the second processor switched on, check pins 2 and 3. Pin 3 is fed through R1 (12 ohms, 1 W), and owners measure about 3.1 V there on working units, so a reading near 3 V on pin 3 is normal.
5. Check for 12 MHz at IC8 pin 12. If there is none, check 5 V at IC8 pins 14 and 7, then the 74S04, the crystal and the parts around them.
6. Boot mode: after releasing BREAK, IC6 pin 9 should be high for a quarter of a second and then stay low. If not, check address line A15 for a short and the reset path to IC26.
7. PHI IN at IC2 pin 37 should run at 187.5 kHz for a quarter of a second after BREAK, then 3 MHz. If it does not reach IC2, follow it back to IC12 pin 13. A collision (IC14 pin 10 not at 0, IC27 pin 8 not at 1) stops the clock.
8. Reset at IC2 pin 40 should go high about 100 µs after BREAK is released. Stuck low points to IC26 or the processor. The ROM chip enable at IC3 pin 18 should be low for a quarter of a second after BREAK.
9. SYNC at IC2 pin 7 shows whether the processor is running: slow pulses in boot mode, faster at 3 MHz. If it stops during boot mode, the ROM is not being selected; if it stops after the change to 3 MHz, suspect the DRAMs or refresh.
10. The refresh request at IC13 pin 15 should be about 68 kHz and should reach IC7 pin 1. RAS (pin 4 of each DRAM) runs at 3 MHz and CAS (pin 15) comes from IC9 pin 8. Acorn warns that the DRAMs may be destroyed if RAS sticks low.
11. If the supplies to IC1 are good and the unit still fails, replace the Tube ULA with a known-good one.

ICs 1 to 3 are socketed; everything else is soldered. A small BASIC program that reads the host-side Tube registers at &FEE0 to &FEEF can show whether a data line is stuck, as hoglet used to find a D0 line that always read zero.

Capacitors

There are only three polarised capacitors in the unit:

Polarised capacitors (service manual pp. 35 and 37)

Board	Ref	Value	Type
Power supply	C1	4700 µF 16 V	Aluminium electrolytic, reservoir
Power supply	C5	47 µF 35 V	Aluminium electrolytic, output
Main board	C3	10 µF 10 V	Tantalum (Acorn part 610,010)

The other main board capacitors are ceramics: C4 to C28 are decouplers (Acorn part 680,002, 33 nF or 47 nF on the circuit diagram), C29 33 nF (the timing capacitor for the reset monostable IC26; the transcribed parts list prints 33 pF against Acorn part 637,333), C30 100 nF, C1 10 nF and C2, C31 to C33 33 pF. Replace C1 and C5 if the supply shows ripple or will not hold 5 V into the 10 ohm test load. See Capacitor Failure Symptoms.

Documentation

• Acorn 6502 Second Processor Documentation: the service manual (0408,003), the user guide (408000) and the main board circuit diagram (108,000 issue 4).
• Acorn's brochure AMP05 and Acorn User's review of May 1984 are on Chris's Acorns.

Related pages

• BBC Micro Model A/B
• BBC Micro Model A/B Troubleshooting Guide
• Acorn 32016 Second Processor