KBK update for Tuesday, 6th July
Recently, I updated my description of two-of-N encoding and in doing so, noted the three different switch arrangements that are suitable for it: dual isolated outputs (e.g. Hall effect), double-pole switches (also isolated outputs) and single-pole switches with the output split in two isolated lines via a pair of diodes. For a while I have been vaguely aware of the Harris HD-0165 16-key encoder—that has been sat in my pending pile of collected advertisements—but not how it worked. Having obtained the specifications and application note, I can now see that it is a 1-of-16 encoder. Essentially it is 16-to-4 binary encoder with input validation and strobe. For larger keyboards, two such encoders must be used together, in a 2-of-32 manner. The Harris documentation confirms that two encoders require double-pole switches or split diode-protected outputs. Unlike discovered TTL two-of-N keyboards that favour 8-input encoders, this is a 16-input variation. Of course, not all 2-of-32 codes are valid: of each input code, only one bit per encoder is permitted, as with any multiple-encoder arrangement.
This is another encoder IC yet to be found in use in a keypad; its only known use to date is in Jameco keypads.
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