Articles in the "Expansion Interfaces" Category

The Microtronix Expandable Interface

The Microtronix Expandable Interface was probably the first third-party alternative to the Radio Shack Expansion Interface for the TRS-80 Model I. It cost $129.95 when it was introduced in late 1978 by Microtronix of Philadelphia, Pennsylvania. According to the product announcement in H&E Computronics, the Expandable Interface was also available in kit form for around $100.00.

Microtronix sold software and hardware for the TRS-80 (as well as the Apple II and Commodore PET), including the Electric Pencil lowercase modification and a 16K RAM expansion. They demonstrated the Expandable Interface at the Personal Computing ‘78 show, which was held in 1978 in Philadelphia from August 25th to 27th. The Expandable Interface had an unusual set of features:

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The Exatron MM+

The Exatron MM+, also known as the Exatron Memory Plus Expansion, was an alternative to the Radio Shack Expansion Interface for the TRS-80 Model I. It was designed and sold by Exatron of Sunnyvale, California for use with their floppy disk storage alternative, the Exatron Stringy Floppy. Despite the fact that it was barely advertised and seems to have been only sold briefly in 1981, the Exatron MM+ remains one of the best remembered Expansion Interface alternatives.

The Exatron MM+ was aimed at users of alternative Model I storage options, such as the Exatron Stringy Floppy, the Meca BETA-80, or the JPC Products TC-8. Unlike the Radio Shack Expansion Interface, the Exatron MM+ didn’t include a floppy disk controller. But it did add additional memory, a parallel port, and a serial port. Unlike the Microtek MT-32, another lightweight Expansion Interface alternative, the Exatron MM+ also extended the Model I expansion bus. This meant that peripherals that connected to the bus, such as the Stringy Floppy or the BETA-80, could still be attached to the MM+.

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The Microtek MT-32

By itself, the TRS-80 Model I was capable of only limited expansion. In order to add memory beyond 16K and support floppy disk drives, Radio Shack sold an external device they called an Expansion Interface.

The Expansion Interface could expand memory to 48K, add a parallel printer port, and add a disk controller for connecting floppy disk drives. But an Expansion Interface with 32K of memory cost $597.00, almost $100 more than a 4K Model I. What if a Model I owner didn’t want (or couldn’t afford) disk drives but still wanted to add memory and a printer port?

The MT-32 from Microtek, Inc. of San Diego, California used a different approach from the Radio Shack Expansion Interface. The MT-32, introduced in mid-1980, offered only two features: the ability to add 32K of RAM (for a total of 48K) and a parallel printer port.

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The Holmes Expansion Mainframe

The Holmes Expansion Mainframe was a popular alternative to the Radio Shack Expansion Interface for the TRS-80 Model I. Introduced in mid-1982, the Expansion Mainframe offered expansion options in a different manner than its competition. It provided extra features through the installation of special plug-in modules, also sold by Holmes Engineering. In addition to supporting the Model I, the Expansion Mainframe was also described as being compatible with the PMC-80 when used with the PMC to TRS-80 adapter sold by Personal Micro Computers.

The Expansion Mainframe was known as a very reliable unit, with gold-plated connectors and all data lines fully buffered. For maximum reliability, Holmes Engineering also recommended gold-plating the connector on the Model I itself using a product such as Gold Plug 80. The Expansion Mainframe was often paired with two earlier Holmes Engineering products, the SPRINTER speed-up board and the Internal Memory memory expansion.

The Expansion Mainframe was housed in an off-white metal enclosure, measuring 16 1/2″ by 9 1/8″ by 2 7/8″, which also served as a base for the Model I monitor. It came in two versions:

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The Lobo LX-80 Expansion Interface

The LX-80 was an alternative to the Radio Shack Expansion Interface that took a different approach to compatibility than the competition. It was originally announced by Lobo Drives International in Fall of 1979, but problems with the supplied operating system meant that it wasn’t released until closer to 1981. The original price was $799 (without memory), although that price had been reduced to $510.00 by late 1982. Lobo Drives also briefly advertised the LX-50, but it is unclear how that differed from the LX-80.

The LX-80 unit was extremely solid (constructed of 1/8″ thick steel) and was designed to have the Model I monitor rest on top. Although more expensive than other Expansion Interfaces, the LX-80 came with an impressive set of features:

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The Micromint Disk-80 Expansion Interface

The Disk-80 was an alternative to the Radio Shack Expansion Interface that was sold by Micromint Inc. It was designed by Steve Ciarcia and was featured in his “Ciarcia’s Circuit Cellar” column in the March 1981 issue of Byte.

The Disk-80 was sold in several configurations:

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The LNW System Expansion

The LNW System Expansion was a popular alternative to the Radio Shack Expansion Interface. Introduced in 1979 by LNW Research, the System Expansion was the most popular of the Expansion Interface replacements. LNW Research later described themselves as “the number one manufacturer of system expansion units and accessories for the Model I computer”.

The LNW System Expansion sold for $69.95 as a kit with a bare printed circuit board and manual. LNW later introduced the $399.95 LNW System Expansion II, which provided the same features but came assembled in a metal case.

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The Radio Shack Expansion Interface

The Radio Shack TRS-80 Model I had few options for internal expansion other than adding a maximum of 16K of internal memory. The external expansion possibilities were reserved for a device Radio Shack called the Expansion Interface.

The Radio Shack Expansion Interface plugged into the back of the Model I using a six-inch long cable and was designed to serve as a base for the TRS-80 monitor. It provided:

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