TNC 1200bps signal into the 9600bps pin

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KJ6HZH, Aug 11, 2017.

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  1. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    Looking at most (all?) VHF/UHF rigs that have a data port, there's one TX_AUDIO pin for 1200 bps and a different one for 9600 bps. I understand that to get 9600 bps, the transmission audio signal needs to be delivered directly into the transmitter varactor diode (quoting from g3ruh's paper) rather than into the microphone input. This seems to suggest that to switch from 1200 bps mode to 9600 bps mode, I would need to switch cables or use a homemade switch box. What would happen if I were to send 1200 bps audio in through the 9600 bps pin?
     
  2. KF5FEI

    KF5FEI Ham Member QRZ Page

    You sure about that? I looked at the manuals of a couple of different radios, and they have 1200 and 9600 baud output pins, but only one input pin.
     
  3. KK4YWN

    KK4YWN Ham Member QRZ Page

    the 1200 bps connector is for audio. the other is for digital communication with the radios processor.

    the 1200 bps gets modulated and sent out over the air. the 9600 bps lets a pc ask the radio what its settings are, and maybe tells it what settings to use.
     
  4. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    Oops.. Yes, I looked again at the manual for my FT8800 and noticed this. So... what if I were to wire the cable such that the TNC listens on the 9600 bps pin for 1200 bps signals?
     
  5. KF5FEI

    KF5FEI Ham Member QRZ Page

    I believe the only difference is the audio on the 1200 baud output is filtered in some way, and the 9600 baud output is direct from the discriminator output. I would check the manual and see if there is a difference in the signal levels or if it explains the difference.
     
    KX4O likes this.
  6. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    I guess the solution is to make a switch box that toggles the output line between 1200 and 9600.
     
  7. KB0MNM

    KB0MNM Ham Member QRZ Page

    Many TNCs also have 'separate' wiring for 1200 and 9600 BPS modes. Remember that modulating at 1200 BPS is considered O.K. on VHF (2M) and UHF; yet at 9600 BPS the modulation is considered inappropriate for VHF under 150 Mhz. This has to do with bandwidth and modulation methods. AFSK ( audio frequency shift keying ) takes less bandwidth, and can be used on some HF bands. Just because you *can use a PC or other source to modulate a radio* does *not* make the output comply with regulations. The suggestion here is that you use a radio which is already set-up for 9600 BPS and 1200 BPS transmissions by the maker, and possibly study the reviews that QST magazine did on these radios a few years ago. Some of the older radios did not do as well as others at 9600 BPS. The other route involves a TNC set-up for any radio. In that case, it is still your job to see that your FM signal deviation is within the limits for the frequency of use ( and that you use an appropriate frequency, generally coordination locally or at least a lookup in a call-book preface ).
     
  8. K7JEM

    K7JEM Ham Member QRZ Page

    Properly done 9600 bps occupies about the same bandwidth as a normal FM voice channel, or a 1200 bps packet channel done over a standard FM radio (AFSK). Both are permitted on 2M.
     
  9. WA9SVD

    WA9SVD Ham Member QRZ Page

    Putting 9600 baud packet leads to horrible distortion. The difference is that the "MIC" input responds to audio frequencies (+/- 300-3000 Hz) and 9600 baud packet does not rely upon audio frequencies alone.1`200 baud packet can be made to work through a "mic" connection (IF the audio level is properly adjusted) but 9600 baud packet can NOT work through the mic. The external aux connector can usually accommodate both types of packet.
     
    KB0MNM likes this.
  10. KB0MNM

    KB0MNM Ham Member QRZ Page

    K7JEM- I agree, that properly done- it is possible to modulate VHF and even HF and get throughput at approx. 9600 BPS. Yet you need to look at the first post in this thread. He was talking about inserting a 9600 BPS audio signal through a VHF port set-up for 1200 BPS. This would be in contrast to using a system such as AOR introduced that uses an external modem to properly modulate an HF transmitter ( also may be used on VHF, etc. ). The answer is that the lower the frequency, the less margin for error ( staying within permissable limits) when using 9600 BPS modulation. So 'properly done' does apply, just not to his question about "What would happen if I were to send 9600 BPS audio through the 1200 BPS pin? " WA9SVD sent a much more appropriate response on August 16th. Hopefully KJ6HZH is reading this mail and heeds the word of WA9SVD. While your answer is technically correct, it fails to look at the original context. The lower 2M band is much more crowded in most areas.
     

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