ROB,
The gates are NAN gates.? A NAN Gate logic is
A? ?B? ? out
========
0? ? 0? ? ?1
1? ? 0? ? ?1
0? ? 1? ? ?1
1? ? 1? ? ?0
For the input, we take the PTT of the Mic direct and invert the Computer.??
When the logic gate is High the diode is off
When the logic gate is Low the diode is on
The only time the computer diode should be forward biased is when Mic PTT is open (logic 1) and the Computer PTT is low (which is inverted to High or a logic 1).? When the gate output is LOW it would turn on the diode to be forward biased.
As stated in the text of the message, if the logic family selected cannot drive the diode, then a transistor or MOSFET needs to be used, and the inverted output needs to be inverted again.? I guess an alternative would be to use a PNP transistor but did not look into that in detail.? I did go back and look at TTL specs, and a standard 7400 quad 2 input nan gate IC should work.
I may have the thinking messed up, but that was what I came up in the spur of the moment.
73
Evan
AC9TU
