Switch between power and signal with MCU
For a programmer you really need to be able to turn on the various control voltages AND have the ability to isolate the pin for insertion and removal. Also, since there are fingers involved, you also need some ESD protection.

As such, at least three control pins per device pin is not unusual.







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Of course, care has to be taken to ensure only one of the inputs is ever active at any one time.

Some devices also have other voltage requirements and a more complex circuit can be required.







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ADDITION

A further must have, or at least REALLY NICE TO HAVE is a sniffer circuit on the Vcc pin that stops the programmer from applying power when the device is inserted upside down. A really good programmer will include circuitry on all pins in case the selected part &lt;> the actual part.

ADDITION 2

If you want do feed A20, wherever THAT comes from, you can use this circuit, ensuring A20 is low when you apply the 12V.







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