Mechanical Enterprises Sabrecoil
Sabrecoil is a switch array system from Mechanical Enterprises: all the switches are formed from a single plastic frame, similar in general idea to Hi-Tek High Profile and Stackpole’s KS-200 and KS-200E. Standard array sizes of 58-position, 64-position, 16-position and 20-position were advertised, but all full-size examples discovered so far are based on a 60-position array. The heyday for Sabrecoil seems to have been 1979–1980, based on current discoveries.
Retrocomputing enthusiast John Honniball (anachrocomputer) possesses a Sabrecoil brochure, but has chosen not to make it available. With no advertisement actually putting the name to the design, one can only assume that the array design covered in US patent 4203016 “Electric switch utilizing coil spring torsion biasing in switch operation” (filed in November 1978) is the design advertised as “Sabrecoil”. A 1980 advertisement that neither depicts nor names the series is taken to be Sabrecoil based on the description; the only fully accessible advertisements naming the series do not depict it either.
An advertisement in Evaluation Engineering in 1981 (partially accessible on Google Books) indicates that Sabrecoil was “introduced two years ago as a standard product for the low-cost, high-volume keyboard market”, putting its date of introduction in line with the aforementioned US patent.
Sabrecoil keyboards use a single plastic frame encompassing all the switches. Each key station requires only four parts: plunger, a pair of parallel coil springs and a keycap. The two springs fulfil the roles of return springs, switch contacts and solder terminals. The bottom of each coil spring extends to form a solder terminal, and the top of the spring is formed into a horizontal bar that is used as a switch contact. As the key is pressed, these bars make contact and close the circuit.
One of the selling points of Sabrecoil was that the switch contacts use silver plate instead of gold, for a lower cost (at 50¢ per station in 1982), bringing with it a longer bounce time; the following explanation was given in Computer magazine, vol. 15, issue 9, September 1982, New Products section on page 107:
Mechanical Enterprises has implemented some trade-offs in order to produce a low-cost, full-travel keyboard. This was accomplished largely by tolerating a slightly longer bounce, which the company feels is acceptable for most applications. More expensive keyboards often use gold contacts to reduce bounce. Mechanical Enterprises’ Sabrecoil uses a specially coated silver plate contact that is “almost as good as gold” but allows a reduction in cost.
Sabrecoil was introduced in the era of microprocessor-driven keyboards, where the longer bounce time could be handled more readily (such as directly within the operating system), as a trade-off for the reduced cost. However, there is some evidence to suggest that Sabrecoil was initially manufactured with gold-plasted contacts. In Product Engineering in 1979, Sabrecoil is advertised as follows (as found on Google Books):
Sabrecoil Keyboard has crossbar contacts that incorporates only three parts for each switch position. Contact noise from gold contacts is minimal; life expectancy exceeds one million cycles per position. Plunger travel is 3.4 mm with overtravel of 1.5 mm; operating force is at 60 grams. Doubleshot moulded keytops provided and keyboard is in a stepped or sloped configuration. Mechanical Enterprises, Inc., 8000 Forbes Place, Springfield, VA 22151. (703) 321-8282, contact is Robert Twyford.
In contrast, an advertisement in EDN (Electrical Design News) from 1980, also found on Google Books and only partially accessible, claims:
Sabrecoil full-keytravel units feature a 1-piece modular construction and employ three parts for each switch position. They achieve minimal contact noise through the use of silver-plated crossbar contacts and sport a tested life expectancy exceeding 10 million cycles per position and 60g switch-to-switch operating force. A simplified pc-board …
It may be that the contact plating was only gold for a year or so before being changed to silver.
The 1982 advertisement indicates that in testing it had reached over five million cycles, with contact bounce still below 15 ms at this point, although the rated lifetime in a 1980 advertisement was 10 million, yet only 1 million when the switch was introduced. Although membrane keyboards can have bounce times as long as 20 ms (such as Alps KFNR Series), mechanical switches are typically rated at 5 ms or less.
The following specifications were taken from a 1979 advertisment in Product Engineering, a 1980 advertisement in Computer Design magazine for an un-named product, and a 1982 Sabrecoil advertisement in Electronic Design. The rated lifetime and contact bounce are not stated as such in the 1982 advertisement, but rather, Mechanical Enterprises noted that after 5 million cycles of testing, the contact bounce still remained below 15 ms.
|Total travel||3.4 mm (1979)|
|Overtravel||1.5 mm (1979)|
|Operating force||60 g (1979, 1980)|
1 million (1979)
10 million (1980)
5 million (1982)
|Contact bounce||15 ms (implied in 1982)|
|Rated load||12 V DC, 0.5 W maximum (1980)|
|Contact resistance||1 mΩ maximum|
Sabrecoil keyboards are not widely encountered. A few examples are now known:
- Acorn used a 60-key Sabrecoil array for the first four versions of the Acorn Atom (Issues 1 through 4). They dropped Sabrecoil with Issue 5 of the Atom (the final version), changing over to Futaba ML discrete mechanical switches in the process. Acorn would continue with discrete mechanical switches for some years.
- AMD specified Sabrecoil for their evaluation kit for the AmZ8000 CPU: a key array matching the design of that used in the Atom (switch frame and the distinctive keycaps) can be seen in their advertisements (see Documentation below). This keyboard appears to be based on the same 60-key array as used by the Acorn Atom, but with the three double-width keys there are only 57 keys in total.
- Powertran’s PSI Comp 80 kit-built computer use two arrays: a 56-key main array and a separate 16-key numeric keypad. The main array is also a 60-position unit, this time with four key stations unpopulated.
All literature scanned by Bitsavers unless otherwise noted.
- Advanced Micro Computers evaluation board advertisement, Computer Design, August 1979, page 182
- Advanced Micro Devices Am96-4016 advertisement, Computer Design, March 1980, page 56
- Modular keyboard advertisement, Computer Design, August 1980, page 208
- Sabrecoil advertisement, Electronic Design, 1982, collected by Marcin Wichary
- Powertran PSI Comp 80 advertisement, Computing Today, June 1979, collected by Flax Cottage
- MEI Sabrecoil on the Deskthority wiki, for prior notes and illustrations