[1000mp] PTT Interface

[jerry]

Hello Carl,
I had the same problem with the PTT on my MP also.   
The MP PTT circuit on the front Mic input  requires a resistance 3900 ohms to key the PTT, not much current required here,  so any opto interface should work.

BUT  .... the PTT inputs on the back of the rig are different,   they require a resistance of 50 ohms or less.  To try to get it to work I tried to lower the resistor on the RTS or DTR line side of the Opto circuit to drive the Opto into mega saturation,  but that caused the RS-232 voltage on the RTS or DTR line to drop from +12 to near zero causing nothing to work.

My fix....

On the cpu side
Go from (RTS or DTR) thru a diode and 2.2k resistor to the base of a 2n2222,  emitter is ground,
collector is going to the Opto thru a 470 ohm resistor then to +12 volts (  here I ran a wire from  a cd or hard drive connector in the cpu.)     All this on the cpu side   for complete isolation.

On the MP side.
the ouput of the Opto to any PTT input on the back.
The Opto can now be driven into saturation and not drop the RTS or DTR at all as that line is only turning the 2n2222 on.
This should work for any Opto, if not just lower the 470 ohm resistor in the input collector a bit.

Another note.......   I drive the FSK and CW key line in the MP  with the TDX and DTR lines of the CPU with no problems using a standard Opto circuit,   just the PTT in the MP requires this added 2n2222 circuit




Best regards. 

jerry    , N2UZ
n2uz@jersey.net
2002-12-26