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American Micro-Systems


Keyboard encoders

AMI are known to have made at least one single-chip keyboard encoder. The article Almost pre-historic computer keyboard lives again claims that “AMI made a series of keyboard encoders in the 1970s but no datasheets for these parts exist today”, with no supporting evidence. Presently only model S9021 is known, from the forum topic Some parallel ASCII keyboards.

Model Keys Modes Bits/key Rollover Output Data Notes
S9021 90 4 10 N-key lockout/NKRO Parallel Mask programmable


S9021 is a 90-key, 10-bit quad mode encoder. The datasheet for S9021 (linked from the aforementioned forum topic) is dated January 1973. The datasheet scan contains a circuit diagram for encoder, in addition to the more typical block diagram.

S9021 has an unusual characteristic of providing a 0.5 V hysteresis region for both the logic and keyboard inputs. This is in addition to supporting the normal practice of a delay period of up to 5 milliseconds to allow the switch output to stabilise.

Electronic shift lock is provided, and the encoder can supply up to 300 µA of current to an indicator lamp for shift lock status.