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Harris Semiconductor


Keyboard encoders


HD-0165 is a rudimentary bipolar logic (instead of MOS) keyboard encoder. Alone, it supports up to 16 keys; combined with a second encoder, it supports 256 keys in total. HD-0165 is not a scanning encoder: it appears to be built around a binary encoder, and every switch must be connected separately to an input line. A single encoder, suitable for a single keypad, functions thus as a 1-of-16 encoder. With larger devices, the two encoders in parallel provide 2-of-32 encoding. There is no ROM look-up table: the customer is expected to wire each switch to the appropriate input (single encoder) or pair of inputs (dual encoders) according to the truth tables presented in the documentation in order to yield the desired output code. In accordance with 2-of-N practice, a dual-encoder keyboard requires dual isolated outputs from the switches, which could be Hall effect, double-pole reed or mechanical, or any single pole switch with the output split through a pair of diodes.

For the most part, the encoder functions equivalently to a 16-line-to-4-line binary encoder. There are two additional outputs specific to keyboard handling: strobe and rollover. The strobe output is active when any key is active; for debouncing, Harris demonstrate how to connect a monostable multivibrator to delay the strobe response until sufficient time has elapsed to clear the switch bounce. For pulse strobe, they show how to connect a second monostable multivibrator to define the pulse length.

Unlike a conventional binary encoder, the HD-0165 incorporates internal clash detection. When two keys are detected simultaneously, the KRO output is active. This line indicates that strobe should be ignored (strobe is not automatically suppressed) and that the output code is invalid. With two encoders, the strobe lines should be combined (Harris show an NOR gate, but NAND seems more reliable) and this line should be suppressed, such as by (NOT KRO) AND STB.

The expected cost per unit when it was introduced around 1971 was $5.

A single HD-0165 was used in the Jameco JE600 Hex Keyboard; the JE664 EPROM programmer contained the same keypad and thus the same chip. A full data entry keyboard based around two HD-0165 ICs is yet to be observed. The same technique was however used in the Zbrojovka Brno Consul 259.13 keyboard using a pair of TESLA MH1KK1 16-to-4–line encoders; here, Hall effect switches were chosen.


All documentation below was scanned by Bitsavers unless otherwise stated.

The HD-0165 specifications and Application Note 204 are both taken from Linear & Data Acquisition Products Volume 1 from 1977, and are thus marked as such. The application note was listed in the Harris CMOS Data Book from December 1974, but it was not included. The specifications themselves will go back much earlier than 1977.