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Stackpole Lo-Pro

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Overview

Lo-Pro, also written LO-PRO, is a range of low-profile keypad switches from Stackpole. Lo-Pro switches use a characteristic oblong housing. The switch contacts are formed from a single coil spring, with a straight projection at each end. Based on their write-up in Electronic Design magazine in November 1972, these switches do not appear to have been intended for data entry keyboards. The Focus on Keyboards article in Electronic Design noted:

The latest Stackpole developments include a mode in which every key can be a repeat key without the need to depress a control key first and the LO-PRO, a new line of low-profile, mechanical keyswitches, designed for calculators and other low-throughput devices.

In Computer Design in August 1975, the following advertisement was placed:

Compact, low-profile keyboards, for calculators, POS devices, and data entry equipment, may be designed with Lo-Pro 5 or wobble-free Lo-Pro 20 keyswitches. Both are available in a high-profile configuration as well. Low-Pro 20 and its high-profile equivalent have integral elastomeric cushions to increase error-free operations to 20 million. Std coding, such as ASCII, is available, while variations can be ordered. Engineering assistance can also be provided.

The “elastomeric cushions” appear to refer to the “electrically conductive elastic members” found in the 1975 patent, which are little caps on the ends of the contact spring. Possibly Lo-Pro 5 is the original 1972 design (with conceivably a five million cycle lifetime) and Lo-Pro 20 would be the 1974 design with an extended 20 million cycle lifetime, although the 1974 design does not seem to have any additional characteristics that would make it more stable.

Where these switches have been found in data entry keyboards, they have been mounted directly to the PCB. However, in April 1974, Stackpole advertised a family of pre-assembled arrays, all plate-mounted. In both discovered keyboards, the switches are suitable for plate mounting, with an overhanging lip and wedges very similar to that of Cherry M4/M5/M6/M7 switches, but have not been used in this manner.

The return spring is external to the switch. Some models have a captive spring, while others use the keycap to retain the spring.

Specifications

Patents

Lo-Pro switches are covered by three patents. The first two depict very tall plunger guide shafts, with the switches having a distinctly tall profile. Those patents also omit the wedge-shaped projections that allow the switches to be plate mounted.

Patent Title Filed Granted Notes
US 3767878 Keyboard switch 1972-09-21 1973-10-23 Tall version
US 3924089 Keyboard switch 1974-08-28 1975-12-02 Covers the elastomer-tipped contacts; tall version
US 4119821 Normally closed switch 1977-09-26 1978-10-10 Short version; only patent to show the plate-mount wedges

Keyboards

Keyboard Date Reference Notes
Unidentified keyboard ca. 1974 Deskthority The keycaps are the same brand of keycap as used by Mechanical Enterprises, and the same typeface.
Unidentified keyboard ca. 1975 Deskthority The PCB is Stackpole-branded but otherwise unidentifiable. The possibly-homebrew converter board uses chips from 1969, 1972 and 1973 while the MM5740AAE/N encoder is from 1975.
Netronics ELF II keypad ca. 1979 cosmacvip.com
Netronics Professional ASCII keyboard — VintageComputer.ca The service manual shows the Stackpole switches but only lists the Controls Research BI-PAC switches in the parts list.

Documentation

All material was scanned for or by WorldRadioHistory.com unless otherwise noted.