First, what little I knew of electrical engineering is now 20+ years old.
Second, as a software guy, I try hard to leave hardware to the hardware
guys. However, sometimes that is hard to do with radio stuff, and now I am
stuck. Any help would be most appreciated.
I am trying to interface my PC running WriteLog to my IC-740. I have the CW
interface working just fine through the LPT port on my computer and the KEY
jack on the radio. Built a little circuit with a 2n22222 (it was easy, I
can solder, and someone else provided the circuit :-), no problem (thanks to
several other WriteLog users for your help).
My next trick was to get the PTT interface working so I could use the PC as
a digital voice keyer. So far, no joy.
First, I built the recommended circuit, which is identical to the CW keying
circuit. It took me a while to figure out that putting out a couple of
millivolts wasn't going to do me any good for PTT - the 740 manual clearly
states that I need to ground pin 3 on the ACC connector.
Ok, so I fool around until I figure out that grounding the emitter, instead
of tieing it to the Strobe (pin 1 on the LPT), made things look much better.
Now when I ask the PC to key the rig, the resistance between the output of
my little circuit and ground goes to basically zero (about .3 ohms by my
meter).
But, when I hook this puppy up to the radio, the strangest thing happens.
When the circuit tries to key the rig, I can hear a relay close in the
radio, and the audio output of the rig is muted, but the transmit light
doesn't come on, and I get no audio out. Manually grounding the output keys
the rig normally. Confusion sets in.
Ok, so I can think of two possible problems on my own:
1) .3 ohms isn't zero - and the radio expects a solid short to ground.
2) I am using two different grounds, and this is causing some kind of
problem. To clarify, when I say that I am grounding the emitter, what I
really mean is that I am tieing the emitter to the ground of the PC. The PC
and the rig are not grounded each to the other. I tried using the ground
from the radio instead of the ground from the PC, but that caused my radio
to always be in this partially keyed state.
I am sure that there are others.
So, I am now in completely over my head. I would appreciate any help,
hints, suggestions, etc. :-)
Thanks much,
***dan
Dan Levin, N6BZA
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