I have tried to make myself a Icom OPC-478 type cloning cable. I found a thread on how to do so here on eHam at:http://www.eham.net/forums/MobileHam/5421?page=2
Specifically, I used the schematic shown here:http://losdos.dyndns.org:8080/public/icom-v8000/opc478-via-max233-mod1_sm.jpg
Using some Radio Shack parts, I turned that into the circuit layout here:http://siriusphoto.com/forum_posts/OPC-478.jpg
I built the thing, verified it looks like the circuit layout and then plugged it in.
Nothing. No function, no smoke. I've poked around a bit with the multimeter, but I've only been able to determine a couple of things. One thing is that the voltage on 7(RTS) and 4(DTR) is negative when the COM port isn't being used by the computer. This makes the voltage on pin 7(VCC) of the MAX233 -5V. I assume when the computer "wakes up" the COM port, the voltages on pins 4 and 7 of the COM port will become positive, thus making pin 7 (VCC) of the MAX233 chip positive and "turn on" the chip. However, this happens so quickly that my multimeter blinks to let me know the voltage is changing, but never stabilizes into a reading before the computer gives up and "turns off" the COM port.
Does anyone have any suggestions for how I might troubleshoot this?