I am looking into connecting a TNC-Pi to a Kenwood TM-241A for AMPRNet operation. I thought it was going to be fairly straightforward as each pin on the TNC-Pi seemed to have its own analogue on the transceiver's end, until I came across this diagram:

TNC to TM-241A connection diagram

I have a few questions about this. For one, why does it use the external speaker output instead of the Rx output pin on the transceiver? Second, why does the TNC connect to the transceiver's ground instead of the microphone input ground? Third, what are those little circles that seem to loop around another wire?

I did notice that this diagram isn't specific to the TNC-Pi, so I might not have to worry about any of this.


3 Answers 3


It likely uses the external speaker output because not all rigs (especially older ones) have an RX audio output pin. And even if a rig does, it's possible that the audio output is before de-emphasis is applied so the TNC wouldn't be able to decode reliably.

As Mike said, the circles represent the cable shield.

As far as the microphone ground, read here: MIC microphone ground vs common (PTT, chassis etc) ground


As you have speculated, the wiring diagram you show is not correct for the TNCPi. Consult page 10 of your TM-241A user manual for the pin-out of your front panel connector and page 10 of the TNCPi manual for the DB9 pinout.

The TNCPi does not have a Thevenin mike ground like the TM-241 does, so you can experiment with connecting the shield for the transmit audio to the mike ground (pin 7) or the chassis ground (pin 8) on the transceiver.

If you connect the audio output from your tranceiver's mike jack (pin 6) to the RXinput of the TNCPi (pin 5), you have the advantage that the audio level into the TNC is independent of the radio's front panel volume control. The radio's speaker will still be active so you can monitor for traffic and you can still turn the speaker volume down when desired.

If you don't obtain enough audio level from pin 6 of your transceiver to drive the TNC, you may have to resort to using the speaker output jack or consider changing R4 (1k) on the TNC to 4.7k to reduce the loading on the signal from pin 6 of the TM-241.

As noted by @mikewaters, the circles in the diagram indicate the use of a shielded cable. The connection to the circle represents the connection of the cable shield.


Those circles represent the shield.

  • 2
    $\begingroup$ Wouldn't this be better off as a comment? It doesn't seem to address the first two questions at all. $\endgroup$
    – Timtech
    Jun 2, 2018 at 4:25
  • 1
    $\begingroup$ @Timtech Partial answers are still answers. Really, there's not a perfect way to post an incomplete but useful answer, but posting it as an answer ensures it can be ranked by votes. Comments should be used for clarifying the question, not answering it. $\endgroup$
    – Kevin Reid AG6YO
    Jun 3, 2018 at 22:31

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