TRS-80 Model I Family

TRS-80 Model I
Manufacturer:	Tandy Corporation
Year Introduced:	1977
Year Discontinued:	1981
Form Factor:	Microcomputer
Word Size:	8-bit
Clock Speed:	1.774 MHz
Memory Size:	4 KB (Level I Basic) 16 - 48 KB (Level II Basic)
CPU:	Z80 (Later Z80A)
Operating System:	Level I Basic, Level II Basic, TRSDOS, Third Party DOSes including LDOS
Successor(s):	TRS-80 Model III
Price:	US$599

The TRS-80 Model I computer, originally known as simply the TRS-80 computer, was released in 1977. It is considered to be part of the "1977 trinity of home computers also including the Apple II and the Commodore Pet. Unlike the 6502 driven Apple and Commodore machines, the Model I used the Zilog Z80 processor running at 1.77 MHz. It spawned two other major models, the Model III and the Model 4 and its variants.

TRS-80 Model I

The original versions came with 4k RAM and Level I BASIC which was on Tiny Basic. In 1978, Level II Basic, a double precision floating point Basic written by Microsoft. Also, in 1978, the Expansion Interface was released. This Interface allowed up to four floppy disk drives and up to 48k RAM in groups of 16k assuming the base system had 16k installed. It also allowed a printer to be attached. Without the Expansion Interface, you could only use a cassette deck as storage.

There were a few different DOSes available with TRSDOS being Radio Shack's OS. LDOS and DoubleDOS were two very popular third-party DOSes.

The second TRS-80 computer, the incompatible business computer Model II, part of the TRS-80 Model II Family, was released in 1979. It should be known that in 1978, a short-lived business system called the Tandy 10 Business Computer was released but it sold extremely poorly and to the best of my knowledge wasn't even featured in the famous Radio Shack Computer Catalogs.

TRS-80 Model III

TRS-80 Model III
Manufacturer:	Tandy Corporation
Year Introduced:	1980
Year Discontinued:	1983
Form Factor:	Microcomputer
Word Size:	8-bit
Clock Speed:	2 MHz
Memory Size:	4 KB (Level I Basic) 16 - 48 KB (Level II Basic)
CPU:	Z80A
Operating System:	Level I Basic, Level II Basic, TRSDOS, Third Party DOSes including LDOS
Predecessor(s):	Model I
Successor(s):	TRS-80 Model 4
Price:	US$699 - US$2495

In 1980, the Model I's mostly compatible successor the TRS-80 Model III was released. It was an all-in-system with a faster and improved Z80A and the ability to type in lower case plus other improvements. Not long after, the FCC declared that the Model I did not comply to new, more stringent RF interference standard and Radio Shack had to take it off the market. Like the Model I, the Model III came in both Level I and Level II Basic varieties although the Level I version wasn't very popular. The most popular system was probably the dual floppy disk system. A new version of TRSDOS, 1.3, was released due to architecture differences. Some Model I disks could be read on the Model III, but not vice-versa. The Model III was in production until being replaced by the more capable Model 4.

TRS-80 Model 4

TRS-80 Model 4, 4P, 4D
Manufacturer:	Tandy Corporation
Year Introduced:	1983
Year Discontinued:	1991
Form Factor:	Microcomputer
Word Size:	8-bit
Clock Speed:	4 MHz
Memory Size:	64 - 128 KB
CPU:	Z80 (Later Z80A)
Operating System:	TRSDOS, Third Party DOSes including LS-DOS, CPM (both 2.2 and 3)
Predecessor(s):	TRS-80 Model III
Successor(s):	Last in its line
Price:	US$1699

The final model in the Model I family was the TRS-80 Model 4 and its variants. Although, it had a Model III compatibility mode, it was much different and more capable machine. First of all and one of the biggest selling points is that the Model 4 is the only descendant of the Model I that could run unmodified CP/M. Where that Model II family could run it from the beginning, the Model I family's memory map was set up in a way that typical CP/M and its software could not be run. Also, being expandable up to 128 KB, it could take advantage of CP/M Plus's bank switching capabilities. The bundled operating system was TRSDOS 6.2, which was essentially a re-badged third-party LDOS 6.2. LS-DOS, an improved LDOS was also available and most hobbyists use it today. There were three variants of the Model 4 and they are described below.

Gate Array Model 4

The gate array model 4 was an improvement upon the system allowing for zero wait-states and full 4 MHz operation of the Z80A. One of its distinguishing characteristics is a green on black display instead of the white on black present in previous TRS-80s.

TRS-80 Model 4P

The Model 4P, released in late 1983, was a transportable Model 4 that unlike the other models lacked a cassette interface. Like many other transportables of its time the keyboard doubled as a lid. It later came came in a gate array version but it is unclear whether the color of the text is different between the two versions.

Tandy TRS-80 Model 4D

The final variant was the Model 4D. It is a gate array system with dual double-sided, double-density 360 KB floppy disks. It was bundled with Deskmate. Also, of note, it was the last computer to bear the TRS-80 nameplate although it shared the Tandy name as well.

Peripherals

In addition, to printers and additional floppy drives (available up to a total of 4 on all models except the Model 4P), 5 and 10 MB hard drives were available. Although, TRSDOS is a flat filesystem, many users made multiple partitions to organize storage.

Legacy

The Model I family is still used by many hobbyists and there have been third-party network cards and even enhanced video playback. SD cards have replaced floppies and hard drives and there are even subReddits and Facebook groups for these systems.

There are many good emulators for the system, but I recommend trs80gp which emulates multiple emulates all of the Z80 TRS-80 systems plus the Model 16, Tandy 6000, and the TRS-80 MC-10, a small, low price home computer. There is also good software representation in the TOSEC for all of the models.

• The site for all things TRS-80 [1]