ad7i@ky2d-2.UUCP (Paul) (09/09/85)
David Anderson:
David -- i am posting this to the net because it might be
of interest to others in addition to just yourself.
The PTT circuits of both TNC1 and TNC2 are almost exactly alike.
I think the only difference is the operating voltage (that might
be the key) and a bypass cap. Given that the PTT voltage, when
you want the transmitter on, is less that 0.5 volts, it sounds to
me like your radio should be keyed up (radio designers don't think
much of my opinion, however).
The problem is that most PTT lines think they are talking to a contact
switch on a microphone -- not a BJT or FET. So, radio designers to
silly things like have the microphone switch short a common emitter
transistor's base to ground to have the radio transmit. That works
well for mechanical switches on microphones but not always for electronic
switches. (I am guessing at this point at the type of PTT switching
inside your radio).
Yes, it would be nice if the TNC2 could switch your radio's PTT but
given that it is not, here are some things to try. I offer several in
hopes that at least one will be useful for you.
1) modify your radio so the when the PTT lead is less than 3 volts
the transmitter will come on. (probably not practical)
2) add a relay either internal or external to TNC2 so that a metallic
contact is provided to switch the PTT lead of your radio
3) try to hand select FETs that have lower saturation voltages.
4) try two 2n3904s in a darlington configuration and remember to add
a base resistor to limit the base current (try 10K).
I hope one of these ideas strikes your fancy and that it works for
you. Please let me know how it turns out.
73, paul ..!ihnp4!hou2h!ad7i (one of the HW developers of TNC2)