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.