The Holmes Expansion Mainframe

written by Matthew Reed

The Holmes Expansion Mainframe

Portion of a Holmes Engineering
advertisement from the August 1982
issue of 80-U.S. Journal

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 II 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:

  • the MF-1, which cost $150.00 and accepted up to four plug-in modules.
  • the MF-2, which cost $99.50 and accepted up to two plug-in modules.

(Some advertisements also referred to a single-module version, but it is unclear if it was actually released.) There were discounts on the MF-1 and MF-2 if plug-in modules were also bought at the same time.

Holmes Engineering sold a number of plug-in modules for the Expansion Mainframe (all prices are from their 1983 catalog):

  • the DX-1S, a single density disk controller (with parallel port) for $148.50
  • the DX-2D, a double-density disk controller (with parallel port) for $219.50
  • the VX-1 VID-80, an 80-column video system for $279.50
  • the RX-232, an RS-232 interface for $99.50
  • the RX-232M, an RS-232 interface plus 32K of memory for $179.50

Also offered was the “Holmes Expansion Interface System,” a package which combined the MF-1, the DX-2D, and the RX-232M, for a combined price of $399.00 (a savings of $150.00 over the individual items).

Holmes Engineering continued to advertise the Expansion Mainframe as late as 1984.

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