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Here is SM6KIN:s
model of an interface. The diagram is primary made for the
transceiver IC 706MKIIG , but it can be adjusted for other models as well.
The interface is combined DC-isolated with Opto coupler and transformer.
The sound board in the PC is isolated by means of an audio-stereo-transformer.
(type Monitor 064370. In Sweden the transformer can be bought from "Kjell & Company"item 39607). Similar stereo audio transformers can also be handled from stores
selling car radio equipment.
The used COM-port signals from the PC are opto isolated , i.e. TxD and RxD used for CAT (CI-V) in both directions, and also the signal DTR used for PTT.
Please observe: For correct CAT connection , it is necessary that either the RTS or the DSR signal (or both), by the software should be put constant high. (+10V DC).
This is easy to set up with MixW and Ham Radio Deluxe. Regarding Digipan the interface function is ok from PC to the rig, but no return signal (on RxD-line) is yet implemented in SW V2.0.
That is: If you run MixW or Ham Radio Deluxe, the frequency at the PC screen will change value when you turn your radio frequency knob, so not in case Digipan. However all the programs will adjust and change band and frequency at the radio when you key a change at the PC/screen, and this is a good thing.
When you have built this device, it is time for a test of the isolation.
Take all antenna and earth connections away from the transceiver. With an ohm meter measure and check that you have full isolation between the chassis of the PC and the Rig!
It is a nice feeling when you run PSK with your expensive units , that you have no earth loop, and that your HF has great difficulties to generate harmonics in your computer cables.
The component costs are low (abt 39 EUR) This audio transformer is handy to use as it has low distortion and two windings in the same enclosure as well as ready audio connectors. The transformer can as shown here be loose, but if you use a bigger box it can be built into the box.
The opto coupler IC: PC849 can be replaced by PC847 (cheaper), but remember:
the 847 has a different pin configuration, so don't forget to change the connection for two of the opto channels!!
(As help you will find a diagram for chip PC847)
Preferably use screened cables and put on some ferrit rings at the cable ends as an extra safety.
I have used a very small plastic box (dim WxH 5x8 cm) to house the PC board.
If you use a metal box, do not in a hurry ground all screens with the inlet screws in the box! Good luck with your interface!!
SM6KIN MAC
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