Acorn BBC Technical Details and Trivia

Connectors

  • a UHF television out port - standard RF socket
  • an RGB video output - 6-pin female DIN socket, for connection to an RGB colour monitor such as those from Microvitec.
  • a "video out" port - BNC socket, outputs PAL composite video in black & white (colour if modified)
  • an RS423 serial port - 5-pin DIN socket
  • a cassette port - 7-pin or 5-pin DIN to suitable cassette recorder sockets (e.g. 5-pin DIN and/or 2.5mm jack socket). NB: 5-pin DIN will not allow the cassette recorder motor to be started/stopped by the computer.
  • an analogue input port, featuring 4 analogue inputs allows the connection of various sensors and other devices - DB15
  • an "Econet" port (optional), the BBC's highly praised network protocol - 5-pin DIN
  • a "User" port, providing 8 digital lines programmable as either inputs or outputs, used for mice, trackballs, digitisers, etc.
  • the "Tube" interface (Model B and up), allowing a second processor and extra RAM to be connected -
  • a power supply socket - +5V @ 3.75A, -5V @ 1A

Technical Facts

  • The printer port on the BBC can be physically re-routed to provide an additional 8 extra I/O lines, just like those offered with the user port.
  • Of the 32K of fitted RAM, up to 20K can be used as video memory. The machine provides up to 7 raster modes (0-6), and one hardware text mode (7). Raster text modes still run with a framebuffer, but save memory by skipping a few rows of the video output after each line of text; the line count of the screen is thus reduced from 32 down to 25, saving 20% of the system RAM used by the framebuffer. In all the raster modes, you can assign any of sixteen colours to the palette entries.
  • Due to the memory limits of the Model A, limited to 16K of RAM from new, video modes 1, 2, 3 and 7 were not available.
  • If your TV has a SCART socket, it is possible that it supports not just composite video, but also RGB video. By connecting the BBC's RGB output to the SCART on your TV will provide the best signal quality if you don't have an RGB monitor. Check your TV's manual for details.
  • The BBC offers eight normal colours, and eight flashing colours where two colours are alternated. For example, colour 9 alternates red/cyan and colour 14 alternates cyan/red. Screen mode 7 uses the SAA 5050 Teletext generator chip to provide a character set the same as what you see in the British Teletext/Ceefax system, including the chunky graphics.
  • The option of whether the BBC uses a 5.25" or 8" floppy disk drive is determined by the PCB link at S10 (this sets whether the index pulse input to the P8271 FDC is taken from pin 4 (5 ¼ inch drive) or pin 8 (8 inch drive) of the connector).

Hover your mouse over the circuit board for a description of the components

Model Codes and Serial Numbers

An Acorn BBC serial number typically consists of

The BBC Model A came in two variants:

  1. ANA01 - Model A
  2. ANA02 - Model A with Econet interface fitted

The BBC Model B came in four variants:

  1. ANB01 - Model B
  2. ANB02 - Model B with Econet interface fitted
  3. ANB03 - Model B with Disk interface fitted
  4. ANB04 - Model B with Disk and Econet interfaces fitted

The BBC Model B+ came in four variants:

  1. ANB51 - Model B+ (64K)
  2. ANB52 - Model B+ with Econet interface fitted
  3. ANB53 - Model B+ with Disk interface fitted
  4. ANB54 - Model B with Disk and Econet interfaces fitted
  5. ANB55 - Model B with 128K

Other serial numbers begin with:

  1. GNB14 - German BBC Model B
  2. UNB09 - USA BBC Model B

 

 

Trivia

  • The BBC could support a second processor via the Tube interface. Options were extensive, including the 6502, Z80, 32016, and 80186 processors.
  • The Econet network facilities were only available if the optional Econet upgrade was fitted inside the machine. "Econet" was so-called to mean "Economy Network".
  • The built-in power supply also has a 5V and 12V output to allow for the connection of external floppy drives.
  • Software began on cassette, but with an optional floppy disk drive and floppy controller upgrade fitted, you could use disks with the BBC. Floppy controller upgrades came in two packages: initially the Intel 8271 controller was used, which allowed what was called DFS (Disk Filing System). This allowed 100K 40-track of 200K 80-track capacity per side of the disk (up to 31 files per side), and filenames up to 7 characters long. Later, the more advanced 1770 floppy disk controller was made available, which provided ADFS support (Advanced Disk Filing System). This allowed filenames to be up to 10 characters in length (as with the cassette interface), and offered a hierarchical directory system on the disk.
  • Support for EPROM s to be fitted directly to the BBC circuit board was not overlooked by both third parties and consumers. One ROM at a time is bank-switched on the fly into an area of the upper 32K of the machine’s 16-bit address space for access. There are four EPROM sockets on the BBC Micro although the OS can address up to 16 "sideways" memory banks, which can each be either an EPROM or a RAM chip. Using an EPROM for software moves the code into the upper half of the address space out of the RAM, and provides instant access to code instead of having to load it from disk. EPROM s could also extend the operating system by way of new OS commands and new filing system drivers.
  • Early BBC Microcomputers, mainly issues 1 and 2, were fitted with a linear type power supply unit which employed a conventional mains transformer and bridge rectifier, the regulation of the voltage being performed by several 7800 series voltage regulators. These power units tend to get rather hot, even in an unexpanded model A machine and do not provide any auxiliary power. The linear supply can be distinguished from the later switch mode type by its black painted case and the absence of an auxiliary power socket.

 

 

 

This page was last updated on 4th February 2015.