Editing Acorn Universal Second Processor Unit

{{Infobox computer peripheral
| name         = Acorn Universal Second Processor Unit
| image        = [[File:Acorn Universal Second Processor Unit.jpg|260px|frameless]]
| caption      = The Universal Second Processor Unit (photo: Chris Whytehead, CC BY-SA 3.0)
| manufacturer = Acorn Computers
| type         = External Tube housing for a BBC Master Series co-processor
| release_date = 1987
| price        = £86.25 including VAT<ref name="app111">Acorn Computers, ''More power for your BBC Microcomputer!'', leaflet APP111, 1987. Hosted on this wiki: [[:File:Acorn Universal Second Processor leaflet (APP111).pdf]]. Purpose, description, technical specification, requirements for each co-processor, prices including VAT and product codes (ANC21, ANB21, ANB28, ANB29).</ref>
| interface    = Tube (40-way ribbon cable)
| compatible   = [[BBC Micro Model A/B|BBC Micro Model B]], [[BBC Micro Model B+]]; BBC Master 128 as an external co-processor
| power        = Built-in mains supply, 220/240 V 50-60 Hz
| model        = ANC21
}}
<templatestyles src="Template:StyledTable/styles.css" />
The '''Acorn Universal Second Processor Unit''' is a second processor case with its own power supply and a motherboard that takes one of Acorn's internal co-processor boards for the BBC Master Series. Plugged into the Tube socket of a BBC Micro Model B or B+, it lets those machines use the Master's 65C102 "Turbo" or 80186 "512" co-processor.<ref name="app111" /> Chris Whytehead notes that it also lets a Master 128 use such a board as an external co-processor.<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/8bit_Upgrades/Acorn_ANC21_Uni2Proc.html "Acorn ANC21 Universal 2nd Processor Unit"], Chris's Acorns, The Centre for Computing History (retrieved 2026-10-02). Serial number 03-ANC21-1000861; photographs open with and without an ADC06 65C102 co-processor; use with a BBC Micro or as an external co-processor with a BBC Master 128.</ref> Acorn's sales code for it was ANC21, and its 1987 leaflet priced it at £86.25 including VAT.<ref name="app111" />

The unit itself contains no processor or memory. The co-processor board, bought separately, carries the processor, the RAM, the boot ROM and its own Tube chip.<ref name="app111" /><ref name="mtug">Acorn Computers, ''65C102 co-processor user guide'', part no. 0443,030, issue 2, April 1986. Hosted on this wiki: [[Acorn 65C102 Co-Processor User Guide]]. Fitting PL1 to SK1 and PL2 to SK2 (pp. 5 to 6); *CONFIGURE TUBE, NOTUBE, INTUBE and EXTUBE (p. 9).</ref>

== Hardware ==

[[File:Acorn Universal Second Processor Unit with 65C102 co-processor fitted.jpg|thumb|right|220px|Lid removed, with an ADC06 65C102 co-processor fitted. The power supply is at the top and the Tube cable leaves at the bottom left (photo: Chris Whytehead, CC BY-SA 3.0)]]

Acorn's specification lists three parts: a motherboard with connections for the co-processor, a built-in power supply for 220/240 V at 50 to 60 Hz, and a cable to the Tube socket. Two rows of sockets on the motherboard take the co-processor board.<ref name="app111" /> The case is the cream "cheese wedge" second processor case also used by the [[Acorn 6502 Second Processor]] and [[Acorn Z80 Second Processor]]; the mains lead and power switch are on the back panel.<ref name="chris" />

The two co-processors available when the unit was launched were:<ref name="app111" />

{| class="wikitable styled-table" style="width:70%; text-align:left;"
|+'''Master Series co-processors for the unit (leaflet APP111; sales codes from Chris's Acorns)'''
! Co-processor !! Processor and memory !! Host requirements
|-
| Turbo (ADC06) || 65C102 at twice the speed of the standard processor; HiBASIC || DNFS, or a 1770 upgrade with 1770 DFS or ADFS
|-
| 512 (ADC08) || 10 MHz 80186 with 512 KB of RAM; runs MS-DOS and CP/M-86 software, supplied with GEM || A 1770 upgrade with ADFS, and a dual 80-track disc drive
|}

Acorn's leaflet warns that some MS-DOS software may not work with the 512 on a BBC Micro because it needs a numeric keypad.<ref name="app111" /> The Turbo board is described on [[Acorn Master Turbo]]. Chris Whytehead gives the 65C102 board's clock as 4 MHz.<ref name="chrisadc06">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/8bit_Upgrades/Acorn_ADC06_65C102CoPro.html "Acorn ADC06 65C102 Co-processor"] and [https://chrisacorns.computinghistory.org.uk/8bit_Upgrades/Acorn_ADC08_80186Copro.html "Acorn ADC08 Intel 80186 CoProcessor"], Chris's Acorns (retrieved 2026-10-02). 65SC102 at 4 MHz; the ADC08 board with an Intel 80186 and 512K RAM, sold with the Master 128 as the Master 512 and running DOS+ and GEM.</ref>

== Fitting and compatibility ==

# Check the host's filing system ROMs against the table above. Acorn notes that Model B owners may need to replace their standard disc filing system with one or more of the upgrades it sold: the DNFS ROM upgrade (ANB21, £20.61), the 1770 upgrade (ANB28, £49.95) and the ADFS ROM (ANB29, £29.99).<ref name="app111" />
# With the unit unplugged, remove the lid and fit the co-processor board onto the two rows of sockets on the motherboard.<ref name="app111" /> On the Master, the 65C102 board's plugs PL1 and PL2 go into sockets SK1 and SK2 and the board sits on support posts; check that every pin enters its socket.<ref name="mtug" />
# With both machines switched off, plug the ribbon cable into the Tube socket under the BBC Micro.<ref name="app111" />
# Switch on and press BREAK. The start-up banner should name the co-processor; the 65C102 user guide lists the messages to expect.<ref name="mtug" />
# On a BBC Master 128, an external Tube device is only used after <code>*CONFIGURE EXTUBE</code>. Use <code>*CONFIGURE INTUBE</code> to go back to a co-processor fitted inside the Master.<ref name="mtug" />

== Fault finding ==

Acorn published no service manual for the unit. Its BBC Master Series Service Manual gives these checks for an internal co-processor, and they apply to the same boards in the Universal unit:<ref name="mastersm">Acorn Computers, ''British Broadcasting Corporation Master Series Microcomputer Service Manual'', part no. 0443,004, issue 1, April 1986. Hosted on this wiki: [[BBC Master Series Service Manual]]. Section 7.12 the Tube; section 7.13 internal co-processor; appendix, PL6 Tube connector pinout (40-way IDC; +5 V on pins 31 to 39, 0 V on odd pins 1 to 29).</ref>

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Co-processor checks (Master Series Service Manual section 7.13)'''
! Symptom !! Checks
|-
| Co-processor not recognised; the host's own start-up banner appears || Configuration (on a Master); the two connectors correctly mated; all connections to the two connectors<ref name="mastersm" />
|-
| Recognised, but only a cursor in the top left corner of the screen || Try the co-processor in another computer; check the two connectors are mated; check the boot ROM or ROMs are correctly inserted in the co-processor<ref name="mastersm" />
|}

Check the ribbon cable and the Tube socket as well. On the Tube connector, pins 31 to 39 carry +5 V from the host and the odd-numbered pins 1 to 29 are 0 V.<ref name="mastersm" /> Owners of the 6502 Second Processor, which uses the same kind of cable, have cured "not seen" faults by fitting a new ribbon cable and by cleaning a tarnished Tube connector.<ref name="sd15087">[https://stardot.org.uk/forums/viewtopic.php?t=15087 "6502 Second Processor not seen"], stardot.org.uk forum thread, May 2018. Elminster fixed similar faults with a new cable and by cleaning a tarnished Tube connector; hoglet diagnosed a data line reading back as zero with a short BASIC program reading the Tube registers.</ref>

Warning: the transformer and the power supply inside the case are connected to the mains. Unplug the unit before removing the lid.

== Documentation ==

* [[:File:Acorn Universal Second Processor leaflet (APP111).pdf]]: Acorn's 1987 leaflet, with the specification, requirements and prices.
* [[Acorn 65C102 Co-Processor User Guide]] for fitting and configuring the Turbo board.
* [[BBC Master Series Service Manual]] for co-processor fault finding.

No circuit diagram or parts list for the Universal unit's motherboard or power supply has been found on Chris's Acorns or archive.org.

== Gallery ==

<gallery mode="packed" heights="200">
File:Acorn Universal Second Processor Unit motherboard.jpg|The motherboard with no co-processor fitted
File:Acorn Universal Second Processor Unit rear panel.jpg|Rear panel, with the mains lead and power switch
</gallery>

== Related pages ==

* [[Acorn Master Turbo]]
* [[Acorn 6502 Second Processor]]
* [[BBC Micro Model A/B]]
* [[BBC Micro Model B+]]

== References ==

<references />

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[[Category:Acorn Computers]]
[[Category:BBC Micro]]