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Isolated ICOM CI-V computer interface
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Reply
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by N4TXI on May 30, 2001
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The following is posted for those who have asked themselves "why do I bother using an isolated audio interface when my unisolated data interface defeats the purpose?":
I noticed there is already an excellent article by KD6UU about making a cheap CI-V interface ("The $10 Icom Computer Interface" http://www.eham.net/articles/1588), so I won't repeat the helpful information he stated there. However, since there seems to be a lot of interest in building isolated audio interfaces here, I thought I'd post the schematic for an isolated CI-V data interface as well.
The schematic is located here:
http://garydion.homestead.com/files/Isolated_CI-V.gif
Basically, the link above shows the meat of the circuit which is centered around two generic optoisolators. I didn't choose them for any reason in particular; I just had them in the shack when inspiration hit. Component values on the left side (computer side) aren't critical, but do keep resistance values over 2k to minimize current drawn from the computer and sent through the LED of the bottom opto. I'd stick close to the values I used on the right side of the circuit (the IC-706 side in my case). By trial-and error, I settled on those values since they resulted in the cleanest data signals (at least they look pretty on an o-scope) while minimizing current draw on the 8v supply rail. I designed the circuit to run on 8 volts since this source is available both on the ACC and microphone plug of my IC-706. The circuit draws a maximum of 0.5 mA from 8v when there is heavy traffic on the data interface. And when there is no traffic? No current draw.
You can see my ugly construction of this unit here:
http://garydion.homestead.com/files/data1.jpg
The green and brown lines go to the CI-V connector - the orange wire goes to a hacked network cable (plugged into the rear microphone jack). This is how I steal my 8v from the Icom.
Any comments are more than welcome - and I can provide a detailed explanation if there is interest. I've been using this interface for a while now and will someday get around to putting it in a box (before field day at the latest). The other connector you may notice in the picture above is a mouse cable connected to the data port on my MKIIG (carries transmit and receive audio, plus PTT.) It is for running PSK31. It's true I could have used the microphone connector instead (since it already powers the data interface) but I didn't think of that before my audio interface was complete. That would probably be a good idea for those of you without the "data" plug. I might still build a combo CI-V/audio box, just for the neatness factor.
73's all - Gary, N4TXI
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RE: Isolated ICOM CI-V computer interface
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Reply
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by AA4PB on July 13, 2002
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Transformer isolation in the audio line is not *totally* defeated by other common ground connections between the radio and the computer. Even with other ground connections, the transformer still prevents ground currents from flowing in the audio shield (which is also the signal return path). It's currents in the shield that cause the majority of audio related problems rather than currents flowing in other ground paths.
Even if you isolate the CI-V and all the audio lines, you may still have several common ground connections between the computer and the radio, depending on the radio, the computer, and the installation. One may be the safety ground on the radio power supply's line plug and the safety ground on the computer's line plug. You may also have a common ground connection via the antenna ground and the computer's safety ground connection.
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