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Interface Cable

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Xfmr Coupling

Three Circuits

The same cable will probably work with all digital modes and software e.g. EasyPal, DIGTRX and UI-View for APRS.  The data interface circuit usually has to do three things:-

1) Connect the radio's receive audio connector to the PC's mike socket.

2) Connect the PC's transmit audio from its headphone socket to the radio's transmit audio connector.

3) Connect the PC's COM port to the radio's PTT line via a voltage converter.

Some radios have VOX or obey CAT commands to switch between receive and transmit.  If so, (3) above won't be required.  It could still be included to make the cable work with other radio models.

Audio Levels

EasyPal generates the required audio data signals from the PC's sound card and they are available at the PC's headphone socket.  It decodes the received audio data signals in the sound card.  There needs to be some volume controls to adjust both of these levels.  In theory, the Windows sound card volume controls can be used for this purpose.  In practice, the volume controls end up at their lowest volume settings and are difficult to adjust.  The cure is to attenuate the audio signals in the interface cable.

Different PCs may give different audio levels at their headphone sockets.  EasyPal, DIGTRX and UI-View generate audio signals from the sound card at slightly different levels on the same PC.  For flexibility in using the data interface cable with different programs and PCs, the resistor attenuators in the interface cable should be variable.  For simplicity, they could be fixed value resistors, but some values may need changing for different PCs or radios.

Variable resistors can be the normal volume control type with a knob rotated by hand.  That type may be accidentally moved and so the audio levels will change.  Preset variable resistors won't be accidentally moved, but require a screwdriver to adjust them. Fixed resistors are smallest and neatest, but may or may not have the correct values to suit different PCs. 

Transmit / Receive Switching

Assuming that the interface cable will be made so that it will work with different models of radio which may not have VOX or CAT, the next problem is that modern PCs don't have a COM port and that is what is expected with most digital programs.  They switch the V.24 (RS232c in the USA) RTS (Request To Send) pin voltage and the interface converts that to a short circuit on the radio's PTT line to make it go into transmit mode.

If you don't have a COM port on your PC, you can buy a USB to COM port adaptor cable.  It has a USB connector at the PC end and a 9 way D plug at the other end.

UK Components Source


Interface Cable With Fixed Resistors

Using resistors in the data interface circuit gives a low component count and should be suitable for desktop PCs.  Laptop PCs with their batteries on charge may introduce hum and noise on the transmitted data signal, making it impossible or difficult to decode.  If that is the case, either the battery charger shouldn't be connected while transmitting or a transformer coupled interface circuit should be used.

If the values shown don't allow the Windows volume controls to be set somewhere near the centre, change R1 for the receive audio level or R3 for the transmit audio level.  Select the correct values before screwing the lid on the box and making things look neat!  I originally used a quad opto-coupler IC as I had many spare.  If you need to buy one, get the single opto-coupler IC.

Notice that only the tips of the stereo plugs have a connection.  Electrostatically screened cables work best when no current can flow along the screens.  The earthy connection is made via the common pin on the COM port.


Interface Cable With Variable Resistors


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Page updated on 31 May 2018

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