1. Principles of operation
The CI-V interface is in use since many years, at least since the 1980s. It uses a single wire as an open collector line to connect two rigs or a rig to a computer. By using an open collector line it is possible that many rigs share a single wire. The number of rigs on a line is not limited in principle, but by the "fan-out" (the driving power) of the units. Icom recommends max. 4 units on a single line. No specifications are made for max. length of the wire, but some tests showed that several meters will work fine. You should use shielded cable in any circumstances.
By connecting two (or more) rigs you get the ability to sync the rigs by frequency and mode. So, tuning one rig always sets the other rig to the same frequency and vice versa. This can be useful for special setups in contests, where you want to receive a single frequency on multiple rigs with different antennas at the same time.
Probably more useful is the connection to a computer:
To connect to the RS-232 serial interface of a computer, a level converter is needed which converts the TTL (5V) level of the CI-V interface to the RS-232 level of the PC. Usually a CI-V converter has more than one connection to a rig (typically four).
This setup allows to remote control the rig with some software on the computer, depending on what the rig and the software supports. If more than one rig is connected to the converter, syncing between the two rigs is still possible. The converter acts as a hub, i.e. the incoming data is sent to all ports (CI-V and RS-232). Some converter designs allow to switch on and off the ports individually.
Due to the single-wire bus design, collisions can occur when two rigs (or a rig and the computer) transmit data at the same time. The protocol offers some limited means to detect and handle collsions.
The CI-V protocol uses single-byte adresses to adress individual rigs on the line. Each model has it's own default adress. If two or more rigs of the same model are connected to the bus, the adresses have to be changed so that each rig has an individual adress.
What is CI-V Transceive?
All Icom rigs equipped with CI-V offers the option of enabling/disabling CI-V transceive. What is this?
With CI-V Transceive set to on, two things happen:
- the rigs transmits data over the bus when frequency or mode changes;
- the rig reacts to data sent over the bus which is adressed not only to it's own adress but also to a special adress meaning "all rigs connected"
With CI-V Transceive set to off, the behaviour changes:
- the rig doesn't send data when you turn the dial or change mode;
- the rig reacts only to data sent to it's specific adress, data sent to "ALL" is ignored.
So in most circumstances you will have CI-V Transceive set to on, at least as long as there is only one rig connected to a PC. If more than one rig is connected over the same interface (converter) to a PC, it might be useful to turn this option off in all but one rig.
Please note that some logging programs require you to turn off the CI-V Transceive function, check the documentation of the software.
Baudrate, serial data format, physical connector
The default baudrate used to be 1200 bps, but most rigs allow for higher speed. Up to 19.200 bps is possible, try to use the highest possible speed. Many newer rigs have an auto option for speed detection, make sure to set all rigs to auto or the same speed, set the PC to the same speed or 19.200 if auto is selected on the rig.
The serial data format is 8 bits, no parity, 1 stop bit.
The CI-V connector is a 3.5mm mono phone plug. Shield is ground, tip is signal. You can use a stereo plug equally well, just connect the ring to ground or leave it open.
Rigs with CI-V and serial ports
Some Icom rigs offer both, a CI-V connector and a RS-232 (aka V.24) port, for example the IC-R75, IC-R8500 and IC-7800. You can use either or both simultaneously. Protocol-wise the interfaces are identical, i.e. they talk the same language. Just that you don't need a level converter for rigs equipped with a RS-232 port. In fact, you can even use these rigs as level converters. What goes into one port (CI-V or RS-232) comes out to the other and vice versa. So if you have one of these rigs with RS-232 and another Icom rig only with CI-V, you don't need another level converter...