Micro Switch 52SD5-1
The photographs below depict an engineering prototype of Micro Switch SD Series keyboard model 52SD5-1. As with all Engineering Keyboard Prototype units, the label does not contain a manufacture date. There are three chips visible on the unit, and these give some idea of the age of the unit:
| Part | Qty | Date | Function |
|---|---|---|---|
| Motorola MC836P | 2 | 7403 | Hex inverter (P = plastic enclosure) |
| SW 20311 | 1 | 7443 | Encoder |
The 1974 chip dates are consistent with our understanding that SD Series was being prototyped in 1974 and entered sale in late 1974 or in 1975. The catalogue listing number’s “5” suffix would imply a card edge connector (which is indeed the case) and US ASCII output, although this is not certain.
The PCB part number is SD-16019, a surprisingly high number for a new series. The manufacturer of the encoder chip is not clear; the chip could be marked “AS” but this is not certain.
The switches are stamped with part number “O O” or “0 0”, instead of a normal SD Series switch code.
The photographs were supplied by Thayer Krantz in September 2021. The unit came from an estate sale years prior, and may have belonged to a former Micro Switch employee.