Digi-Mode Interface


GM4KGK DIGI-MODE INTERFACE  (Updated - 13 Sep 2005)

This interface was put together when I became a little more serious about PSK31 operating. It's use is not restricted to PSK - it can be used for any mode that requires an interface between a transceiver and a soundcard. It has now been upgraded to include CAT control of a transceiver although this section can be omitted if not required. Likewise, the CAT controller can be built on it's own as the two sections are independent of each other, but integration of the two units tidies up the connections between the PC and the transceiver.

A circuit diagram (schematic) appears at the bottom of this page.

The soundcard part of the interface completely isolates the earths on the PC and the transceiver and eliminates ground loops. To achieve this it is essential that there are no connections between the PC ground and the transceiver ground at the box which houses the interface. The simplest solution is to use a plastic box! Please note that the addition of CAT control has resulted in the loss of complete isolation as the CAT ground has to be referenced to the MAX 232 chip which in turn has to be connected to the PC ground. This could be overcome by incorporating more opto-isolation but the additional complexity is hardly justified.

All the components are readily available from component stockists and none are critical to the operation of the interface. Although T1 & T2 are shown as having impedance ratios of 20:1, any audio transformer will work in practice and they don't need to have the same ratio as long as you are prepared to play with some of the resistor values. Make sure that the low impedance side of T2 goes to the MIC connection if you feeding the rig via the front panel microphone socket - this is essential to provide the required attenuation. If audio is fed to the rig through a rear panel socket (the recommended option) you will usually need much more gain and it may even be necessary to reverse the input and output connections to the transformer. T1 is not quite so critical, but you will have more control over the system if the low impedance side also goes to the radio (AF output from transceiver).

The RTS signal is taken from the COM port on the PC to activated the PTT line on the transceiver. The opto-isolator (IN25) works very well and is far superior to VOX operation which is not really recommended for PSK.  The RTS line may be disabled by opening SW1 if the PTT function can be implemented under CAT control. The switch is not essential but some programs assert the RTS line and this permanently keys the transmitter.

VR1 & VR2 are not strictly necessary either, but front panel controls save the hassle of bringing up the Mixer Control Panel to make any level adjustments.

The CAT control circuitry is based on the ubiquitous MAX 232 chip and requires only a handful of components and a power supply (non-critical 8-15v at a few mA's). No attempt was made to "steal" some voltage from the COM port - most shacks are not short of the basic minimal supply required. Please note the polarity of the tantalum capacitors (C1-C4).

The CAT system has been tested with both Icom and Yaesu transceivers - for Icom rigs the SI (Serial In) and SO (Serial Out) connections are strapped together. R1 had to be included as the Serial Out line on my FT-990 requires a pull down resistor for the open emitter output. It has no detrimental effect when used with Icom transceivers.