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.

Model II

Tandy saw businesses as the primary market for its computers. The Model II and its software were especially designed for such customers,^[4] and only Tandy-owned stores sold the computer.^[5] 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. The different disk format and system architecture made it impossible to run Model I/III software on the Model II, thus it never had as much available. This was somewhat mitigated by the availability of the CP/M operating system for the Model II from third parties such as Pickles & Trout. Unlike the Model I/III, the Model II's memory map is compatible with standard CP/M. Three internal expansion slots could be used for add-on cards such as additional serial ports and a video board that allowed bitmap graphics.

BASIC was provided on disk rather than in the system ROMs, partially to give the Model II a more professional and less of a home-computer appearance. The Model II BASIC was similar to Level III Disk BASIC on the Model I/III, but had cassette support removed and incorporated several enhancements from the then-latest version of Microsoft BASIC-80, which included enhanced string-handling features, an improved editor, octal number conversion, and error-trapping. On the downside, the PEEK and POKE commands were removed from Model II BASIC which made it less hacker-friendly. However, if CP/M was used, the user had access to MBASIC for CP/M which was very similar to Model II BASIC and did allow PEEK/POKE.

The floppy drive included with the Model II was a Shugart SA-800 full-height, single-sided 8" drive; like most such drives, it spun continuously whether the disk was being accessed or not and the spindle motor was powered directly off the A/C line. The floppy controller in the Model II was a double-density, soft-sector unit based on the WD 1791 floppy controller. Although the Model II otherwise used double-density (MFM) diskettes, TRSDOS OS disks utilized a single-density (FM) boot track. CP/M distros did not use FM boot tracks. CDC drives were used for the floppy expansion module.

There were several hardware revisions to the Model II over its lifespan. The earliest models could not boot from a hard disk and the floppy controller required a terminating resistor pack for the last drive on the chain, which was not needed on later revisions. Hard disks offered for the Model II also used a terminating resistor pack; these were sold as master and slave drives, with the master hard disk (which had the resistor pack) needing to be the last one on the chain. Like most hard disks offered on 8-bit computers, there was no subdirectory support and the drive was simply treated by the OS as a giant floppy disk.

Unlike the Model I/III, the Model II also had a case fan due to the heat generated by the 8" floppy drive's continuously running spindle motor. The combined effect of the case fan and the floppy motor resulted in an extremely noisy computer compared to the nearly silent Model I/III.

The video display in the Model II is similar to the Model I. A 12" RCA B&W television CRT is used. However, the Model II's video circuitry was significantly improved in the interest of better picture quality, as one of the criticisms of the Model I was that the included monitor was merely an RCA television set with the RF, IF, and sound stripped out. The Model II, in contrast, used a dedicated monochrome composite monitor with higher-quality and better-adjusted components than the modified TV set provided with the Model I. The text display on the Model II was 80x24 rather than the Model I/III's 64x16 text and also added lowercase letters, one major feature that the Model I was lacking. In addition, it could be operated in 40x24 text mode. The character set in the Model II was somewhat different from the Model I/III. It de-emphasized graphics characters and replaced them with additional currency and mathematical symbols.

Despite being designed primarily for business or operating factory equipment, the Model II did have a handful of games available; notably the Scott Adams Adventure series were offered for it. In addition, CP/M versions of Infocom text adventures could be run.

Some of the technical advances first introduced on the Model II such as the WD 1791 floppy controller and the improved video circuitry would later go into the Model III.

The Model II architecture theoretically supported up to 512K RAM via a bank-switchable upper 32K page segment (up to fifteen 32K pages were supported).^[6] 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 $3,350 and the "primary unit" 8.4Mb hard disk another $4,040 by mail-order from Radio Shack's dealer in Perry, Michigan; MSRP in the company's own stores was higher.^[7]

Model 12

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.^[8] 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.

Model 16

Tandy in February 1982 released the TRS-80 Model 16,^[9] as the follow-on to the Model II; an upgrade kit was available for Model II systems. The Model 16 added a 6 MHz, 16-bit Motorola 68000 processor and memory card, keeping the original Z80 as an I/O processor. It had two half-height ("thinline") double-sided 8-inch floppy drives, though the Model II upgrade did not replace the floppy drive.

The Model 16 could run either TRSDOS-16 or TRS-Xenix, a variant of Xenix, Microsoft's version of UNIX. TRSDOS-16 was 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.^[10][9]

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.^[10] A rumor stated that Tandy would offer Unix for the computer.^[11] In early 1983 the company indeed switched to Xenix, and offered it for free to existing customers;^[9] by mid-1983 an estimated 5,000 of 30,000 Model 16s ran Xenix.^[12] Xenix was based on UNIX System III, also supported up to three users, and was more established.^[13] 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,^[14] 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.^[15] In 1987 Tandy announced that the 6000 hardware would no longer be improved; customers believed that their systems had become orphaned technology.^[16]

Reception

InfoWorld in 1981 called the Model II "a well-designed, capable business system" that "overcomes several limitations of the Model I".^[1] 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.'"^[2]

BYTE in August 1984 described the TRS-80 16B as "a usable multiuser microcomputer system", but with a slow hard drive that might limit the computer to two users.^[17]

References