Which Jack for HF Radio-to-TNC Cables?

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Many HF radios have more than one jack that can be used to cable the radio to the TNC:

Front panel microphone jack (headphone or speaker jack may also be needed)
Rear panel accessory jack
Rear panel FSK jack
Rear panel Key In jack

The signals that need to be wired are Transmit, Receive, PTT, and Ground. Some signals may be on more than one jack, but only one wire is necessary for any one signal. For example, a pin for the Receive signal may be on the microphone, headphone, speaker, and rear panel accessory jacks, but only one pin needs to be wired because all receive pins provide the same signal. The same is true of PTT and Ground (earth or chassis ground).

The transmit pins are not all the same. Depending on the digital mode, there are different methods that can be used to generate the Transmit signal: Audio, FSK (frequency shift keying), or CW (continuous wave or Morse code). The mode you place the radio in will determine which pin is used to generate the Transmit signal. In SSB mode (by convention all digital modes on all bands are LSB), the Audio pin is used; in Rtty mode, the FSK pin is used; and, in CW mode, the CW pin is used.

The Audio transmit pin is found on the microphone jack, and may also be on a rear panel accessory jack. When using voice, a person speaks into the mic and the Transmit signal is modulated with the person's audio. For all digital modes except CW, the TNC generates audio tones, which will be sent to the radio if the Audio transmit pin is wired. These audio tones will modulate the Transmit signal, just as a person's voice modulates the signal, when the radio is in SSB mode. This method of transmitting is often called AFSK (audio frequency shift keying).

The FSK transmit pin is often found on an RCA-type rear panel jack (or it may be on a rear panel accessory jack). Wiring the FSK pins between the radio and TNC creates a simple contact-switch circuit. The TNC opens and closes the switch. While the radio is in Rtty mode and the switch is open, the radio transmits a tone; while the switch is closed, the radio transmits a different tone. (Old radios shift the carrier frequency instead of generating two tones.) The FSK transmit method can be used for two-tone digital modes such as Amtor, Gtor, Packet, Pactor, and Rtty. Most radios generate tones that are 170 to 200 Hz apart; therefore, 300 baud packet can be transmitted by this method, but not 1200 baud packet. (Not all radios have FSK capability.)

The CW transmit pin is often found on a rear panel jack labeled "Key In" (or it may be on a rear panel accessory jack). Wiring the CW pins between the radio and TNC creates a simple contact-switch circuit. The TNC opens and closes the switch. While the radio is in CW mode and the switch is closed, the radio transmits a carrier; while the switch is open the radio does not transmit. This transmit method is only used for CW mode. If VOX (voice operated transmit, also called full break-in or QSK) is used while in the CW mode, the PTT wire is not required.

Deciding which transmit method to use will narrow your choices of which jack to use. Other considerations for each jack are listed below.

Front panel microphone jack

Audio transmit method - Radio's SSB (LSB) mode
No need to worry about room noise getting into the transmitted signal.
TNC output drive level is probably set correctly. If not, it can be adjusted. You can also use the mic gain control to adjust the ALC (automatic level control) for a low meter reading. (Remember to re-adjust it for voice.)
Probably no narrow receive filters. Most digital modes use a narrower bandwidth than voice. With voice filters, there is normally a lot of extra noise received with the digital signal. To get rid of some of the noise, you can use passband tuning or add an external DSP audio filter.
If the mic jack does not have a pin for the Receive signal, a separate wire will be needed (usually wired to the headphone or speaker jack).

Rear panel accessory jack

Audio transmit method - Radio's SSB (LSB) mode
If the microphone is connected, room noise may get into the transmitted signal. Some radios may mute the microphone input when the TNC asserts PTT on this connector.
TNC output drive level will probably need adjusted. The rear panel jack usually requires a high drive level. Radios are designed so that a very weak signal on this pin will not be transmitted. This allows you to keep the TNC cable plugged into the radio when operating voice. If there are small noise fluctuations on the cable, they will not be transmitted with the voice signal. On some radios you could use the mic gain control to adjust the ALC (instead of adjusting the TNC), but you will need to remember to re-adjust it for voice.
Probably no narrow receive filters. Most digital modes use a narrower bandwidth than voice. With voice filters, there is normally a lot of extra noise received with the digital signal. To get rid of some of the noise, you can use passband tuning or add an external DSP audio filter.

Rear panel FSK jack

FSK transmit method - Radio's Rtty mode
No need to worry about room noise getting into the transmitted signal.
TNC output drive level and radio mic gain control are not applicable. The TNC can not over- or under-drive the Transmit signal, because the radio is generating the Transmit signal based on the open or closed circuit to the TNC.
Narrow receive filters. Most radios will have filters built around the standard 2125 Hz and 2295 Hz tones used for digital modes. Other radios may give you a choice of filters.
Radio and TNC tones need to be the same. The Receive signal is sent from the radio to the TNC as tones. The TNC normally expects the tones it receives to be the same as the tones it transmits. However, when using FSK the tones are created by the radio, not the TNC. Therefore, the TNC needs to know which tones the radio is transmitting. Many TNCs are designed to only decode the standard 2125/2295 tones (or close to those tones). Other TNCs, like the KAM and KAM Plus, will decode the tones set by their MARK, SPACE, and SHIFT commands.
Mark transmitted on open or closed. The TNC and radio need to agree on which tone is transmitted when the FSK circuit is closed; and which tone is transmitted when the circuit is open. This is especially necessary for Amtor and Rtty/Ascii modes. The TNC manual should tell you if the switch is opened or closed when the TNC sends a mark. (The two tones are normally called "mark" and "space".) Some TNCs have an FSK invert command that will reverse the state of the switch for mark. Some radios may have a switch setting for sending mark or space when the circuit is open or closed.

More information

Transmit, Receive, PTT, and Ground signals, are discussed in Chapter 5 of the book What is your TNC doing? Chapter 5 Table of Contents | Book Table of Contents
Filters and tuning an HF digital signal, are discussed in the article Tuning HF Digital Signals.
Radio-TNC Wiring Diagrams Index

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