Every once in a while, I get the urge to build a computer interface for my IC-735. This isn’t as easy as it sounds as the Icom CI-V interface is not a standard RS-232 type interface. The signal levels are TTL-level signals and it’s a two-wire interface.

The last time I got the urge, I did a web search and came up with the following:

• The Icom CI-V Interface. This is a complete online manual to the interface and the software driving it. The page on CI-V level converters has links to many different circuits, including a relatively complicated one using the MAX232 chip.
• G3VGR Icom CI-V Interface. This is a nice little circuit using a couple of 2N2222s, two diodes, and four resistors.
• THE ICOM CI-V COMPUTER INTERFACE. This is an article written in 1986 describing the CI-V interface. It not only describes the physical interface, but the command set as well. One thing I didn’t know–the IC-735 was the first rig to use this interface.
• Ham Radio Controller Programs. This page contains links to a bunch of rig control programs and information, not solely for the IC-735.
• 801SCAN. 801SCAN is actually a control program for the R7000, R7100 or R9000 receivers, but this page also has a circuit for a RS-232 to CI-V interface. This circuit uses a 74HC04 IC. Do not try using a 7404 in place of the 74HC04. Also, I would advise against using his construction technique, to wit, “Chop the unused pins off of the 74HC04 and glue it to one side of the DB-25 shell with super-glue…” This is NOT good amateur practice :) .
• CI-V Icom/PC Interface OK1DUO. This circuit uses a MC145407 IC, which I’ve never heard of. It also requires a couple of zener diodes. It’s certainly much more complicated than the G3VGR circuit above.
• Icom CI-V Interface Design by K4PWO.This design uses a MAX233 RS-232 interface IC and a 78L05 voltage regulator. There is also a sample PC board design. It’s a nice design, but I don’t think it will fit in a DB9 shell.

It’s odd that I didn’t find any that use optoisolators. I’d think that doing this with optoisolators would be ideal. I guess the problem is that the rig doesn’t supply any power since it’s only a two wire interface (signal and ground). So, even if you use an opto, you won’t be isolated because the interface is supplying the voltage.

Of course, I could go out and buy an interface, but I think any self-respecting ham should build something like this that is so simple.