2. Hardware

2.2.1. PTT

By default, radios are in receiving state, so you have to indicate when to transmit. Note that this is also true for full-duplex radio (which can transmit and receive at the same time). The line responsible for this task is the PTT (Push-to-Talk), usually activated at low-level (Ground) and deactivated at high-impedancy.

There are many ways to interface the PTT, the phonepatch provides applications for the two most common alternatives:

• Serial

  Use a serial port (pin RTS) to set/unset PTT.

  Figure 5. PTT signalling using a serial port

  

• Parallel

  Use the parallel port (pin DATA0) to set/unset PTT.

  Figure 6. PTT signalling using a parallel port

  

These schemes have been taken from the Soundmodem page. Soundmodem is an interesenting project that uses soundcards as modem for digital transmission, so the hardware involved is similar to what we need for building the phonepatch hardware.

You must find the PTT (located usually at the rear part of the radio) and connect it to your serial or parallel port depending on what solution is best for you.

After that, you should test that PTT works well, using the utility radiocontrol:

radiocontrol -v -m serial -d /dev/ttyS0 -p on
radiocontrol -v -m parallel -d /dev/parport0 -p off
radiocontrol -v -m command -d eboard -o "unset ptt, set ptt,," -p on

2.2.2. Carrier detection

UHF/VHF transceivers work usually with FM modulations, which means that they have a way to determine if a radio is receiving audio or not: that's called carrier detection. When using such transceivers, apart from the PTT signalling the phonepatch must poll the state of that signal, to decide whether the audio received comes from a remote perr or not. If detected, this audio will be sent to the phone link.

As for the PTT, the carrier detection is made with the serial and parallel port:

• Serial

  Use a serial port (pin DCD) to get carrier detection state

  Figure 7. Carrier detection using a serial port

  

• Parallel

  Use a parallel port (pin Busy) to get carrier detection state

  Figure 8. PTT signalling using a parallel port

  

After that, you should test that carrier detection works well, using as well the utility radiocontrol:

radiocontrol -v -m serial -d /dev/ttyS0 -c
radiocontrol -v -m parallel -d /dev/parport0 -c
radiocontrol -v -m command -d eboard -o ",,get carrier,get carrier: (1|0)" -c

2.2.3. Device lines fully configurable

Although lines used by default on these board are pretty common (RTS, DCD, DATA0, ...), your cable may use others for PTT keying and carrier detection (and power needed for carrier detection). For this reason, all lines (both serial and parallel port) are fully configurable with the option --device-lines pttline,carrierdetectionline,powerline. In this example we use DTR for PTT, DCD for carrier detection and RTS for power:

radiocontrol -v -m serial -d /dev/ttyS0 --device-lines dtr,dcd,rts -p on
radiocontrol -v -m parallel -d /dev/parport0 --device-lines data,paperout,autofeed -c