Jump to page content

Invac Corporation and Digitronics Corporation

United States



Invac Corporation (typically styled as “INVAC”) was a communications equipment manufacturer based in Massachusetts, with their location variously given as East Natick and Waltham. The fate of the company is not known. Products from Invac include keyboards, paper tape punches and paper tape readers. In particular, Invac claim to have invented the original photoelectric keyboard. In reality, the concept of a photoelectric encoder keyboard appears to go back nearly ten years before Invac, although Invac’s design may have been the first full alphanumeric photoelectric keyboard.

It would appear that Invac Corporation were acquired by Digitronics Corporation based in Albertson, New York, possibly between 1967 and 1969. The PK-200 Series flyer was amended by 1969 to use a combined Digitronics/Invac logo, and magazine advertisements from 1969 onwards were in Digitronics’ name rather than Invac’s name. The most recent Invac advertisement was from 1969. PK-144 and PK-164 were listed in Computer Design magazine in 1965 as Invac keyboards, while PK-275 was listed in a 1969 Terminal State-of-the-Art Survey as a Digitronics keyboard; see under listings of available products for details.


Encoded typewriter

US patent 3032163 “Data recorder and associated methods” filed in November 1960 describes a means of adding photoelectric sensing to a manual typewriter, and may have been Invac’s starting point. Their intentions included “[avoiding] the use of complex mechanical matrices or the like which are employed in conventional data recorders such as tape punches” and “to provide for adapting presently available types of apparatus, such as typewriters, for tape punching operations”. The paper tape punch would be operated from the photoelectric sensors within the typewriter.

The Invac UNICOM/30 brochure from 1966 depicts the use of typewriters as terminals, which are presumably of the photoelectrically-encoded system developed by Invac.

PK-144 and PK-164

Around the same time, Invac also developed a dedicated photoelectric keyboard. This is described in US patent 3092310 “Data processing apparatus”, filed in August 1961. A model matching the patent drawing has yet to be seen. The earliest known dedicated electronic keyboards are models PK-144 and PK-164; these are 45-key and 63-key models respectively. The PK-144 and PK-164 brochure (see Documentation, below) does not give a date, with the only visible code suggesting 1960. PK-144 alone was advertised in Electronics magazine in August 1962: see Documentation, below. No details are given in the advertisment, but the Manual Input Devices section of Computer Design from December 1965 provides some details about their operation. Both models were intended for interactive computer consoles as well as tape punching. Each key generated an 8-bit output code, although binary codes up 18 bits long can be generated.

These models are indicated to have no contact bounce as a result of photoelectric sensing; this was significant at the time as keyboards had not yet introduced solid-state debouncing techniques that would become trivial once matrix scanning was affordable.

Due to the risk of invalid output with two keys pressed at once, such operation is prevented with a physical lock-out. A tray of ball bearings is placed below the key levers, with the total width occupied by the balls being short of the tray capacity by the width of one key lever. As the a key is pressed, the ball bearings are pushed aside by the key lever. All the unused space in the tray is now consumed, and the balls will block a second key lever from being pressed.

Additionally, there is a risk that the key will not be pressed far enough, or will return too quickly. Thus, once the actuation travel is reached, a pair of solenoids will press the key fully and lock in position until the output code has been successfully read.

Series PK-200

Series PK-200 expanded on the design, offering full customisability. Output codes could be up to 14 bits long, with 10 bits as standard; it is not clear whether this means that the shutters themselves allowed up to 14 bits. Keyboards could be ordered with any number of keys between 10 and 75. Key layout and operating force could be customised, and additional controls could be mounted into the case. The mechanical interlock to prevent simultaneous keypresses remained present. The lamps for sensing were wired in series, so that if a single lamp failed, the keyboard would stop outputting text, rather than output incorrect data. This appears to have been true of PK-144 and PK-164, based on the brochure.

The Series PK-200 flyer (see Documentation, below) gives three standard models PK-244, PK-264 and PK-275, with 44, 64 and 75 keys respectively.

Despite the use of mechanical interlocks, the peak character entry is cited as being 20 characters per second.

There is only one discovered example of Series PK-200, a 49-key PK-244–based card punch keyboard with a pair of Burroughs B-8971 15-segment alphanumeric Nixie tubes (rare in their own right), along with two banks of toggle switches. Xah Lee depicts another keyboard of the same model on his website, that was demonstrated at an enthusiast meet-up. Back in 2016, another example was sold on eBay, and while all the listing photos are now lost, Google still retains the thumbnails, where the model of “PK-244” can be read off the identification plate:

This unit has reappeared on eBay; see below.


Model Details Reference
PK-244 49-key unit with control panel with dual Nixie tubes; model number unconfirmed KBref
PK-244 49-key unit with control panel with dual Nixie tubes ∑ Xah Code
PK-244 49-key unit with control panel with dual Nixie tubes; same ultimate source as the unit above eBay


All material was scanned by Bitsavers unless otherwise noted.