Microtan history

The Tangerine Microtan 65 was first sold in 1979. Its philosophy was expandability. The basic system sold as a kit for £59.95 plus £1.50 p&p and had a 6502 running at 0.75 MHz, 1 KB of RAM, 1 KB of PROM, a television interface (16 lines of 32 characters) and a keyboard interface. The system monitor, TANBUG, was held in the 1 KB PROM and provided commands to view and modify memory, and to run and debug programs. Later revisions held TANBUG on a 2 KB EPROM. Similarly priced systems available at the time—the Acorn System 1 and Sinclair MK14—used seven-segment “calculator” displays instead of television output and were not as expandable.

This basic system could be expanded to suit your needs and funds:

• The keyboard could be either a 20-key hexadecimal keypad or a full ASCII keyboard.
• Lower-case characters.
• “Chunky” 64×64 graphics.
• A case!

To expand further, you needed a backplane. Two were available:

• Two slots: one for the CPU board and one for the Tanex expansion board.
• Eleven slots: one for the CPU, one for Tanex, one for TanDOS and eight general-purpose slots.

The first expansion board was Tanex:

• Up to 7 KB of RAM.
• Six EPROM sockets.
• Two 6522 VIAs.
• A serial interface with RS-232, TTL and 20 mA current loop.
• A tape interface.

It was possible to buy a Microtan 65 CPU, small expansion backplane and an expanded Tanex as a complete system. This was known as a Micron.

EPROMs available from Tangerine included:

• XBUG, a monitor extension providing tape save/load, assembler and disassembler.
• Microsoft BASIC.

There were also many third-party EPROMs: word processors, games, BASIC extensions and more.

One other variation on the system appeared in Electronics Today International in 1980. This was a Space Invaders machine designed around the basic Microtan board, with additional hardware to generate the sounds. ETI’s sister magazine, Computing Today, also had an article giving the game listing—which could be loaded into the RAM on the Tanex board—and the sound hardware, which could be plugged into the keyboard socket.

The listing appeared as a raw hexadecimal dump covering nearly two sides of A4. Back then, it took me two days to type it in and debug it; it was easy to confuse “8” with “B” and “0” with “D”. After writing the emulator, I scanned the original pages, tidied them up and used OCR on them. I got the whole thing working in under two hours!

Tangerine sold the rights to the Microtan to Microtanic Computer Systems so that it could concentrate on its Oric system. Microtanic continued to develop additional cards for the system for a number of years. In the end, there were many boards available for the eight general-purpose slots, developed by Tangerine, Microtanic and other third parties:

• High-resolution graphics: 256×256 monochrome, with three cards capable of being synchronised to give RGB output.
• Sound: one or two AY-3-8910 sound chips.
• Banked EPROMs.
• TAN-DOS floppy disks, using a controller card with its own operating-system ROM, DBASIC record-file routines and board RAM.
• The Mousepacket Colour VDU, providing a 384×250-pixel display and supported by later TANBUG and BASIC firmware.
• Additional and alternative CPUs; the 6809 was popular and had FLEX as its DOS.
• Real-time clock, including a Microtan construction project published in Electronics Today International in April 1983.
• Colour 64×25 teletext-type display interface.
• EPROM programmer, which appeared in Electronics Today International.

The surviving hardware and expansion manuals include reconstruction documentation for TAN-DOS and the Colour VDU, the Mousepacket video-board documentation and schematics, and the ETI real-time clock project with its MM58174A application note.