TRS-80 Model II
The TRS-80 Model II was a computer system launched by Tandy in October 1979, and targeted at the small-business market.
Despite its name, the Model II was not an upgrade of the original (Model I) TRS-80, but an entirely different system.
The Model II was succeeded by the compatible TRS-80 Model 12, Model 16 and Model 16B and the Tandy 6000.
As a professional business machine, the Model II used state-of-the-art hardware and had numerous features not found in the primitive Model I such as DMA, vectored interrupts, a detachable keyboard, and port instead of memory-mapped I/O. It sported 80x25 text and a singled-sided 500k 8" floppy drive, and either 32 or 64k of RAM, along with two RS-232 ports and a Centronics-standard parallel port. The video memory could be banked out, so that the whole 64k address space could be used for main memory. Unlike most computers, it had no BIOS ROM except a small boot loader (the BIOS was loaded off the boot floppy). Because of this and the use of port I/O, almost all of the Model II's memory could be used by software. The Model II ran the TRSDOS operating system (renamed to TRSDOS-II starting with version 4.0) and BASIC. TRSDOS for the Model II was not compatible with TRSDOS for the Model I; thus the Model II never had the same breadth of available software as the Model I (a situation aggravated by the fact that Tandy discouraged third-party software development). This was somewhat mitigated by the availability of the CP/M operating system for the Model II from third parties such as Pickles & Trout. Three internal expansion slots could be used for add-on cards such as additional serial ports and a video board that allowed bitmap graphics.
The Model II architecture theoretically supported up to 512K RAM via a bank switchable upper 32K page segment (up to 15 32K pages were supported). However, the machine did not provide enough card slots to physically upgrade the RAM to 512K. This was because RAM was provided via 32K or 64K cards and only a few open card slots were available on a standard Model II since the basic configuration of the machine took up 4 slots.
Tandy offered a desk custom-designed for the Model II for US$370. It could hold an additional three 8" disk drives or up to four 8.4Mb hard drives (the Model II allowed three external floppy drives to be daisy-chained to it). In 1981, the 64K Model II computer was $3350 and the "primary unit" 8.4Mb hard disk another $4040 by mail-order from Radio Shack's dealer in Perry, Michigan; MSRP in the company's own stores was higher.
The Model II was replaced in 1982 by the TRS-80 Model 12, which used half-height ("thinline") double-sided floppy drives, and integrated most of the Model II electronics into a single main board. The video/keyboard card plugged into a single slot in the main board. An expansion card cage was available as an option, allowing more plug-in cards. The Model 12 was essentially a Model 16B (described below) without the Motorola processor, and could be upgraded to a Model 16B.
Tandy in February 1982 released the TRS-80 Model 16, as the follow-on to the Model II; an upgrade kit was available for Model II systems. The Model 16 adds a 6 MHz, 16-bit Motorola 68000 processor and memory card, keeping the original Z80 as an I/O processor. It has two half-height ("thinline") double-sided 8-inch floppy drives, though the Model II upgrade did not replace the floppy drive.
The Model 16 can run either TRSDOS-16 or TRS-Xenix, a variant of Xenix, Microsoft's version of UNIX. TRSDOS-16 is essentially a TRSDOS II-4.1 application providing a 68000 interface and support for up to three users, with no additional features and little compatible software. 68000 functionality was added as an extension, loading 68000 code into the 68000 memory via a shared memory window with the Z80.
The Model 16 sold poorly at first. By June 1982 the company had shipped 2,000 units to stores with the majority not sold. Five months after its introduction the computer still had no TRSDOS-16 applications; owners had to run Model II software and applications. A rumor stated that Tandy would offer Unix for the computer; in early 1983 the company indeed switched to Xenix, and offered it for free to existing customers. Xenix was based on UNIX System III, also supported up to three users, and was more established. With Xenix, the Model 16 family became a popular system for small business, with a relatively large library of business and office automation software. Tandy offered multi-user word processing (Scripsit 16), spreadsheet (Multiplan), and a 3GL "database" (Profile 16, later upgraded to filePro 16+), as well as an accounting suite with optional COBOL source for customization. RM-COBOL, Basic, and C were available for programming, with Unify and Informix offered as relational databases. A kernel modification kit was also available.
TRS-Xenix was notable for being a master/slave implementation, with all I/O being performed by the Z80 while all processing was done within the otherwise I/O-free 68000 subsystem.
Model 16B and Tandy 6000
The Model 16 evolved into the TRS-80 Model 16B with 256 KB in July 1983, and later the Tandy 6000, gaining an internal hard drive along the way and switching to an 8 MHz 68000. Tandy offered 8.4MB, 15 MB, 35 MB, and 70 MB external hard drives, up to 768 KB of RAM, and up to six additional RS-232 serial ports supporting multi-user terminals. Additional memory and serial port expansion options were available from aftermarket companies.
The 16B was the most popular Unix computer in 1984, with almost 40,000 units sold. Internal variants of the Model 16 architecture were built running at speeds in excess of 10 MHz, 68010 processors, up to 8Mb of RAM, SCSI disk interfaces, and up to 12 RS-232 ports.
InfoWorld in 1981 called the Model II "a well-designed, capable business system" that "overcomes several limitations of the Model I". Creative Computing in 1984 called it a "state-of-the-art business machine" that "might have taken the business market by storm had it not had a nameplate reading 'Radio Shack.'"
- TRS-80 Model II Technical Reference Manual. Fort Worth, TX: Radio Shack. 1980. p. 75.
- Kilobaud Microcomputing (magazine), November 1981, page 57, "TRS-80 Discount", Perry Oil & Gas Incorporated 137 North Main St. Perry MI 48872 (advertisement).
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