Tangerine Microtan 65

The Tangerine Microtan 65 (sometimes abbreviated M65) was a 6502 based single board microcomputer, first sold in 1979, that could be expanded into, what was for its day, a comprehensive and powerful system. The design became the basis for what later became the Oric Atmos and later computers. Those later machines have similar keyboard addressing and tape I/O as the Microtan 65. The Microtan 65 has a hardware single step function that can be used for debugging software in both ROM and RAM. The computer was available as ready built boards or as kits consisting of board and components requiring soldering together. The Microtan 65 was intended as a general purpose microcomputer which could be used by laboratories, Original Equipment Manufacturers (OEMs) 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. 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 books for the Microtan system and subsequently for the Oric range of computers.

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).

The Microtan 65 was designed as a modular system able to be expanded as required, and for this each board included an 80 pin connector at one end allowing it to be plugged into a backplane type motherboard. A simple two socket "Mini Motherboard" connected the Microtan 65 and TANEX boards for minimum expansion and in this form the system was also available ready built from Tangerine, complete with case and full ASCII keyboard, as the Tangerine Micron, costing £395.00 in 1981. For further expansion the builder could purchase the full "System Motherboard" which featured an additional ten sockets, bringing the total available sockets to twelve. For housing this, a "System Rack", rack based case was available, in black and silver with a black front panel trimmed in Tangerine's trademark orange. Additional boards became available with time, including a 40K memory board TANRAM, made up of 32K of dynamic and 8K of static RAM, bringing the total non paged memory to 48K, a dedicated parallel I/O board featuring 16 parallel input/output ports; a similar dedicated serial I/O board featuring 8 serial input/output ports, a disk controller board for use with disk drives, a Disk Operating System TANDOS 65, a high resolution graphics board featuring 8K of static graphics RAM giving a resolution of 256 × 256 pixels, a dedicated 32K ROM board, (aimed mainly at OEM and general purpose applications or for use with AIM, KIM and SYM systems), capable of holding either 8 × 2732 or 16 × 2716 EPROMs; and a 32K RAM board featuring two 16K banks of 4116 dynamic RAM, again intended for similar purposes to the 32K ROM board. In addition, several third party suppliers offered boards designed for use with the Microtan 65 system. In addition to the BASIC programming language Tangerine also released on disk TANFORTH, an extended version of FIG FORTH featuring a full FORTH compiler and editor.