Tangerine Microtan 65
Tangerine produced one of the first 6502-based kit computers, the Microtan 65. It had a 3U form factor, a small amount of memory (RAM), a video character generator and UHF modulator for use with a TV set, and a simple latch for entering hex data from a keypad, and the computer was designed to be expandable. The manual came with a one-kilobyte listing of Conway's Game of Life. An optional expansion board could be built with a UART, more memory and BASIC ROMs. Additional expansion boards became available later, offering more RAM, dedicated serial and parallel I/O boards, etc.
After the Microtan 65, Tangerine planned to build a desktop machine and managed to get as far as selling the design for the Microtan 2 also known as Tangerine Tiger to HH Electronics, better known for building amplifiers. They released it as the HH Tiger, but it was not a commercial success.
The Microtan 65 in the full System Rack enclosure and with the ASCII keyboard
The Microtan 65 was intended as a general purpose microcomputer which could be used by laboratories, Original Equipment Manufacturers (OEM)s and the computer enthusiast, and it was designed with expandability in mind. In this way the customer could customise the system, be it as a specialised control system, as a learning tool, or as a general purpose computing device.
Price of the Microtan 65 board in 1981 was £79.35 (inc. VAT) in kit form or £90.85 ready-assembled. The system was not generally available in the shops, one of the few stockists at the time being Henry's Radio of Edgware Road, London.
To accompany the hardware and to offer further support to users, a magazine was created, the Tansoft Gazette (name inspired by the Liverpool Software Gazette). This was edited by Tangerine employee Paul Kaufman who continued as editor when the magazine was renamed Oric Owner. Tansoft also became the name of Tangerine Computer's official software house which supplied a number of software products and book for the Microtan system and subsequently for the Oric range of computers. The Tansoft Gazette was prepared in-house using very basic layout facilities and then printed by local printer, Mid-Anglia Litho. Oric Owner improved on this by using a professional typsetting company.
Release date 1979
The major advance that the Microtan 65 had over a lot of the competition at that time was that the video display was flicker free. At the time a lot of microcomputers would either access the screen memory asynchronously to the video timing (causing flicker and splats on the screen), or would write to the screen memory during a non-display period (which was slow). The Microtan 65 got over this problem by making use of an incidental feature of the 6502. The 6502 (unlike most other CPUs) has a regular period in each instruction cycle when all CPU activity is inside the chip, leaving the external memory available without using complex external arbitration logic. This made video display design simpler and meant that video accesses could be made at maximum speed. This technique is also used on the Oric-1 and Atmos, and in the unrelated Apple II.
The 32×16 characters was the reason that the 6502 was clocked at 750 kHz. To get the circuitry to work at a (nearly) standard video rate meant that the pixel clock had to be 6 MHz. When the Microtan 65 was designed only a 1 MHz 6502 was available, and so 750 kHz was used (6 MHz divided by 8).
Our Microtan 65 system is a home customised system as can be seen by the picture but with the original Microtan keyboard and was very kindly donated by Chris Owens
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This exhibit has a reference ID of CH23946. Please quote this reference ID in any communication with the Centre for Computing History.