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(Revised March 1, 2007)While there are many ways to make a keyboard scanning circuit, the important part here is to send the proper signals to the LEFT and RIGHT logic circuitry: a 6-bit binary count (A0-A5), a logic '1' for a pressed key (DATA), and a low-going strobe during the 2nd half of each change of the Least Significant Bit (/STROBE -- called CLOCK* in the Oberheim schema). Although, cheesy as it is, a CD4024 ripple counter would probably be good enough (it was good enough for Oberheim,) I've revised my revision to now use a CD4520, but not because it is a synchronous counter. Both 4-bit counters of a 4520 have CLK and ENable inputs, which are somewhat interchangable: if ENable is used as a clock input, then the CLK input becomes an enable/disable.
I realized this feature would be necessary in order to be able to adjust the Digital-to-Analog Converter. By temporarily jumpering the DATA out from the lower 4051 to the CLK (used here as an enable) of the 4520, a pressed key (logic '1') will stop the count, causing the DAC to stay at that one value, instead of its normal continually incrementing operation. The trim pots in the DAC can then be adjusted for the proper lowest and highest voltages.
(Some simpler mono keyboards actually work this way, stopping the count and also latching it so the voltage is held when the count continues upon key release.)
The Q1a output of the 4520 is inverted to create /STROBE and Q2a becomes the LSB.
The CD4051 analog muxes here are always enabled (INHIB = 0): nothing happens in the LEFT-RIGHT note/voice assignment logic until /STROBE goes low and count and DATA will be valid by then. In fact, we want the key state to be ready and waiting when /STROBE=0 happens.
Basically, the matrix Rows are groups of eight keys, and the Columns are each key within those groups. For an excellent and more thorough explanation of keyboard scanning and the diode matrix, see the designs at Music from Outer Space.