Master 128

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This page is about the Master 128 and offers information about hardware features common to related machines in the Master Series. You may also have arrived looking for the Master 512, Master Turbo, Master ET or Master Compact.
The BBC Master 128
Acorn seems to have hit the nail well and truly on the head with the Master. There will no doubt still be the sniggerers laughing at the use of eight bit technology – just as they address the memory question I kicked off with. However, my answer to them would always be 'It's not what you've got, but what you can do with it', and for my money you can do a good deal more with a Master than with any other micro around in its class at the moment.
Acorn User, 1986[1]

The British Broadcasting Corporation Master Series Microcomputer is the successor line to the Model A, Model B and Model B+ released by Acorn Computers. The base Master 128 model is an evolution of previous BBC Micro models offering various refinements over the older machines.


As early as 1984 Acorn were aware that the BBC Micro, powerful and successful computer though it was, required improvement to remain competitive in the market. Schools - one of Acorn's main markets - were more or less saturated with Acorn products, while in an industry that was fixated with memory capacity the BBC Model B's 32K seemed feeble in comparison to computers with double this RAM as standard, with the 128K years soon around the corner.

Acorn's initial response was the Model B+ but soon after this machine's release word that Acorn was working on a totally new kind of BBC Micro began to spread. While this new machine was popularly referred to as the 'Model C' in the Acorn press, Acorn themselves at first used the rather inappropriate codename 'Project B'. The tentative name 'Master 64' was released in 1985, with the final name being changed to reflect the computer's standard 128K of memory.

Technical specifications

These specifications are based on those supplied in Acorn promotional literature.[2][3]


Width: 476mm

Depth: 346mm

Height: 79mm


8-bit 65C102 running at 2MHz clock frequency


64KB main RAM

64KB sideways RAM consisting of four 16K pages


128KB onboard ROM


35K Machine Operating System with extended graphics and Terminal software


16K Edit program and text editor

13K View 3.0 word processor

16K Viewsheet spreadsheet


16K 1770 DFS

Internal ROM sockets

2 x 128 or 256Kbit capability

1 x 128Kbit capability

Total sideways ROM or RAM usable at any time 256K including 96K fitted firmware

Cartridge slots

Master Cartridge
Master Cartridge Rear

2 enhanced Acorn cartridge slots

Internal 1MHz bus uprated to 2MHz bus speed

32K ROM capacity per socket

Sound input/output

With replacing ROM software on a standard BBC Model B being a tedious job involving considerable dismantling and risk of damage to the chips' pins, Acorn offered a more convenient solution in the form of the Master's cartridge slots. The twin slots are based on similar technology to that employed in the Electron's Plus 1 upgrade.
Acorn seemingly intended to release software in cartridge form as they had done for the Plus 1 (some of whose cartridges can run on the Master), but it appears that only Logo, OverView and ISO Pascal were released in this format. The slots are more usually used to hold cartridges such as those made by Acorn and Care Electronics which could carry standard 28 pin ROMs.
ROMs connected via the cartridge slots are recognised as numbers 0 to 3 in the Master's ROM map.

Disc Interface

Shugart standard

MFM, double data density

FM, single data density

Supports 40 or 80 track drives with a 6ms step rate or better

Formatted capacity 320KB - MFM, 80 track, per surface total 1.28M on twin 80 track double-sided drives

34 way IDC connector

Network Interface

Acorn Econet


5 pin DIN socket

Printer Interface

8-bit Centronics compatible

26 way IDC connector

Serial Interface

RS-423 75-19200 software selectable baud rate

Independent transmit/receive baud rate selection

5 pin 'domino' DIN socket


8 standard screen modes and 8 shadow modes providing the same displays without affecting user memory.

Graphics commands extend colour range by colour mixing.


Mode Colours Graphics Text
MODE 0 2 640x256 80x32
MODE 1 4 320x256 40x32
MODE 2 8 solid colours and 8 flashing colours 160x256 20x32
MODE 3 2 N/A 80x25
MODE 4 2 320x256 40x32
MODE 5 4 160x256 20x32
MODE 6 2 N/A 40x25
MODE 7 N/A 40x24 Teletext


Phono socket

UHF channel 36 full colour

BNC connector

Composite video 1V peak to peak, monochrome or colour with modification.

6 pin 'domino' DIN socket

RGB TTL level/+5V/positive or negative sync


4 channels with full software control

5cm internal speaker

User Port

10-bit memory mapped bi-directional TTL compatible port

+5 volts available

20 way IDC connector

1MHz Bus

General purpose bus extender

Audio input and output

34 way IDC connector



Custom interface for the connection of second processors

40 way IDC connector


Custom interface for the connection of co-processors

2x 12 way internal connectors

Internal or external Tube priority selectable by software

The Master 128 could still use the 'cheese wedge' external Second Processors originally released for the earlier BBC Micro models thanks to the retention of the external 40 way Tube interface. In addition internal connectors on the motherboard could support an internal processor board, termed a Co-Processor to distinguish it from the earlier external boxes.

Analogue Input

4 channel analogue to digital conversion

8 bit accuracy

1.8V reference voltage

Light pen strobe connection to CRTC

Accepts external reference voltage for higher precision

15 way D-type connector

Cassette Interface

CUTS standard with 300-1200 software selectable baud rate

Output 200mV peak to peak

Input 50mV to 5V

Motor control relay 1 amp at 24 volts DC

7 pin DIN connector

Real Time Clock

Battery back-up Lithium cell, minimum 1 year life

The rechargeable lithium cell was later replaced with three standard alkaline AA batteries. This configuration is by far the more common one. See below for more details.

Information can be called from MOS, BASIC and other languages



64-key QWERTY keyboard with 2 key rollover and auto repeat (rate and delay software selectable)

10 function keys

19 key numeric pad

Screwdriver-operated BREAK key lock

The Master's keyboard is largely similar to that of the Model B. The differences include the addition of a numeric keypad to the right, the changing of the arrow keys to a more logical layout, and the shifting of the red function keys to the left to accomodate a hidden switch to disable the BREAK key. This last feature was added purportedly due to inadvertent operation of the key in schools, to which many students fell victim at some point.

Auxiliary power socket

+12 volts

+5 volts

-5 volts

Power available is dependent on internal options fitted.

Issue differences and variations

Unlike the Model B, the Master changed surprisingly little during its lifetime. There are only two motherboard versions and very few differences between them, with the issue 2 remaining the staple for the Master's entire life.

Very early machines used a rechargeable battery that fitted in the recess next to the cartridge slots to power the battery-backed CMOS. Allegedly due to embarassing incidents of Masters erupting in flames due to reverse current charging, Acorn changed this to regular AA cells, some in a re-usable battery box, others in a disposable shrink-wrapped pack placed next to the keyboard.

The connector at PL12 on the west side of the motherboard, originally designed with an internal modem in mind, is either nonexistent, a row of pins or a socket. The Master AIV or Domesday Machine requires the socket version for attachment of the internal SCSI board.

Some Masters have the system ROM soldered to the motherboard rather than socketed. This is uncommon.