After the initial overwhelming success of the TRS-80 Model I in 1977, Tandy-Radio Shack quickly decided to capitalize on their good fortunes and proceeded with the design and development of a serious business class computer. Their initial offering was the Tandy 10 which was built by Applied Digital Data Systems and released in 1978.
The Tandy 10 was ultimately unsuccessful and Tandy soon thereafter released the TRS-80 Model II. Targeted at the small business market, the Model II was announced in May 1979 and released later that year. The Model II was considered a successful design and became a workhorse system as an accounting, word processing, etc. platform for many small to medium sized businesses. The architecture of the Model II would continue to be developed over the next 6 years in different iterations, including the TRS-80 Model 16, TRS-80 Model 12, TRS-80 Model 16B and finally, the ultimate incarnation of the Model II, as the Tandy 6000. This group of machines will be referred to as the TRS-80 Model II Family.
TRS-80 Model II
The Model II was delivered as a 4 Mhz Z80 based microcomputer system. The standard configuration was typically 64K RAM, although 32K was an option. The Model II had advanced features compared to the Model I, such as Direct Memory Access (DMA). The architecture did theoretically support RAM up to 512K in a 32K bank switchable operation. However, very little software took advantage of this feature. VisiCalc was the only significant title to leverage more than the standard 64K RAM.
The Model II was designed with expandability in mind. Taking a cue from the S100 architecture of the day, the Model II contains a card cage with 8 total slots sharing a common 80-way bus. The Z80 and related IO subsystems are all provided on removable cards. A standard Model II contained 4 cards: a Z80 CPU card, a Floppy Disk Controller (FDC) card, a Video/Keyboard controller card and a 32K or 64K memory card.
Theoretically, you could run any microcomputer platform in the Model II as long as the functionality was provided on compatible cards which could interface with the Model II bus. This feature was to be utilized to run XENIX and other MC68000 based operating systems in the Model 16 and other subsequent iterations of the Model II Family. There was even a prototype 8086 board by Veritas which apparently never made it to or was unsuccessful in the market.
The Model II came with a single full-height single-sided double-density 8 inch floppy disk drive. The Shugart SA800 disk drive was used primarily during the Model II run with the Texas Peripherals (TPI) clone of the SA800 released later in the Model II lifetime. Tandy offered a disk expansion system which allowed up to 3 additional 8 inch floppy disk drives to be attached to the system. The expansion bay was outfitted with full-height Control Data Corporation (CDC) disk drives until late in the Model II run. As in the the computer itself, TPI drives replaced the CDC drives late in the Model II lifetime.
The Model II provided 2 serial ports and 1 parallel printer port. The serial ports could be used for RS-232 or BiSync serial communication with compatible peripherals, such as modems.
The Model II was initially released with TRSDOS 1.x for the Model II as the operating system. This was soon replaced with TRSDOS 2.0 for Model II to address many bugs and deficiencies in 1.x. Several variants of CP/M were released for the Model II. These include Pickels & Trout CP/M (P&T), Lifeboat CP/M and Aton CP/M. P&T became the most popular CP/M variant and is considered by many to be the defacto CP/M for the Model II.
Tandy also released a 8 inch hard drive which provided 8MB of storage capacity. This required a new operating system, TRSDOS-II 4.0. A hard drive adapter card was provided that plugged into one of the empty slots in the Model II card cage. This adapter interfaced with a controller board that was integrated into the hard drive.
The video system of the Model II provided 80×24 characters display. There were a number of graphical characters available that were used to create tables and basic charts. A high resolution graphics board was released which provided 640×240 monochrome graphics. This board would plug into the Model II bus and would integrate directly with the standard video/keyboard board utilizing ribbon cables. There was apparently no Tandy software released that utilized the graphics board.
A Model 16 upgrade kit was made available for the Model II when the Model 16 was released. This consisted of a MC68000 CPU board, a 128KB 68000 RAM board and an upgraded power supply. This allowed a Model II to run TRSDOS-16 and early versions of TRS-XENIX.
An ARCNET card was also made available late in the Model II run. ARCNET was a hub-spoke networking system which failed to find success on the Tandy platform.
More to come about the Model 16, 12, 16B and 6000!