Oak Full Travel Membrane
Full Travel Membrane—commonly abbreviated by Oak to “FTM®”—is a type of membrane keyboard design from Oak. FTM is one of the oldest membrane keyboard types, being introduced around 1980. Although Datanetics introduced their batch-fabricated array keyboards at the end of the 1960s, those keyboards used a PCB for the bottom layer, while FTM is an all-membrane design. The initial (and possibly only) design is dual-spring-over-membrane, using a large outer return spring and a small internal membrane pressure spring.
Oak claimed to achieve N-key rollover entirely in software, without needing to use diodes. Their approach to this was dubbed Entry Error Elimination, or E³. This is not N-key rollover in the modern sense; it appears to be what would be classed as two-key rollover now, with blocking under ghosting conditions.
In a write-up in Electronic Design magazine in September 1980, the conductive material was said to comprise “20-Ω/sq carbon with 20-mΩ/sq silver to provide good adhesion at low cost.”
Oak FTM used several different switch designs. Detailed disassembly photos exist of the first version. Much less information exists for the other versions.
“Version 1” is the switch type demonstrated in US 4367380 filed in 1980. In common with other spring-over-membrane keyboard designs, the original Oak FTM uses two coil springs. A small return spring inside each switch applies operating pressure to the membrane assembly when a key is pressed. An external return spring rests on the switch body and is held captive by the plunger: removing a keycap does not release the return spring. The alternate action cam track is present on the plunger of all switches. See Oak FTM: Switch Dissection on Flickr for a detailed depiction of the switch.
The specifications below are taken from two datasheets, one for Oak FTM Keypads & Switch Arrays from October 1983, and one for Oak’s IBM PC Work-Alike I and II keyboards from December 1985, both scanned in by TG3 Electronics. The differences in force and travel are to be expected, but it’s curious to note a 25% reduction in switch lifetime with the later specifications. This could have been a cost-reduction measure on Oak’s part; the 100 million cycles rated lifetime was offered from the start in late 1980.
|Characteristic||1983 FTM datasheet||1985 Work-Alike datasheet|
|Rated lifetime||100 million||75 million|
|Travel||0.160″ nominal (ca. 4 mm)||0.150+0.000−0.010″ (3.81+0.00−0.25 mm)|
|Pretravel||0.080±0.25″ (ca. 2±0.6 mm)||0.080±0.20″ (2.03±0.51 mm)|
|Actuation force||2.2±0.5 oz (ca. 62±14 gf)||1.9±0.2 oz (53.2±5.6 gf)
3.2 oz (space bar)
|Keystem angle||0° or 11°|
|Maximum number of keys||231||N/A|
|Contact bounce||500 µs typical; 5 ms for “hard rap induced bounce”|
|US 4367380||Keyboard assembly and components therefor||1980-08-27||1983-01-04||Depicts the flat-top, cross-mount version, including alternate action|
|US 4420744||Keyboard crosspoint encoder having N-key rollover||1981-02-12||1983-12-13||N-key rollover implementation suitable for membrane keyboards|
All documentation was scanned by Bitsavers except where otherwise noted.
- Full-Travel Membrane write-up, Electronic Design, September 1 1980, page 211 (from Marcin Wichary)
- Full Travel Membrane technology review, Computer Design, October 1980, pages 82 and 84
- FTM Entry Error Elimination advertisement, Computer Design, March 1981 page 238
- 81-key FTM keyboard advertisement, Computer Design, May 1982
- FTM advertisement, Computer Design, May 1983, page 61
- FTM Keypads & Switch Arrays datasheet, October 1983 (scanned by TG3 Electronics)
- Low-profile FTM advertisement, Computer Design, June 15 1984
- IBM PC Work-Alike I and II datasheet, December 1985 (scanned by TG3 Electronics)