A History of the Icom CI-V Bus (Work in Progress)
Icom's CI-V bus is a single conductor that carries information transmitted using asynchronous serial communication at TTL voltage levels; bus drivers must be open collector. Is original purpose was to enable separate transmitter and receiver products to function as a transceiver, but has evolved to support the monitoring and control of Icom transceivers by computer-hosted applications.
The simplest topology consist of two nodes using a CI-V bus to communicate: a transceiver, and a CI-V bus interface connected to a computer serial port. More than one transceiver can be connected to a CI-V bus, each with a unique CI-V address. Some amplifiers, antenna controllers, antenna switches, and antenna tuners can track a transceiver's frequency if connected to the that transceiver via the CI-V bus.
Collisions can occur on an Icom CI-V bus for the same reason that collisions can occur on an Ethernet segment: two nodes send messages that overlap in time. Both protocols specify a means by which sending nodes detect collisions and warn receiving nodes, receiving nodes ignore damaged messages, and sending nodes retransmit damaged messages.
Every Icom HF transceiver since the IC-735 has provided a CI-V port on its backpanel. A computer equipped with a serial port connected to a CI-V bus interface could send commands to a microprocessor in the transceiver to report or change the transceiver's frequency and mode. Over time, the elements of transceiver state that could be monitored and modified has grown: split, rx/tx status, receiver passband characteristics, S-meter readings, etc. This functionality is referred to as transceiver control. Frequency-sensitive devices like tuners, amplifiers, antenna switches, and antenna controllers could be designed to connect to the CI-V bus in order to passively monitor the transceiver frequency.
When computers began including USB ports, Icom added a the ability to directly connect a transceiver to a computer with a standard USB A-B cable, eliminating the need for both a serial port and a CI-V bus interface. Icom provided a Device driver that when installed in our computer created a virtual serial port (as distinguished from an actual physical serial port interface. A transceiver control application like Commander can be configured to interact with an Icom transceiver either through a physical serial port and CI-V bus interface, or via a virtual serial port created by the device driver when the USB A-B cable connects the computer and transceiver. Note the backpanel CI-V port remained present; an Icom transceiver equipped with a USB interface could still be controlled via a physical serial port - and frequency-sensitive devices like tuners, amplifiers, antenna switches, and antenna controllers could still track the transceiver's frequency by connecting to the backpanel CI-V port -- even if transceiver control is being accomplished by a virtual serial port created by the device driver and A-B USB cable.
Development of the PSK31 mode and the MMTTY application led many hams to connect (via an appropriate interface) the transceiver's audio input to a computer soundcard's output, and their transceiver's audio output to a computer's soundcard's input so that computer-hosted applications could transmit and receive digital mode signals. Icom (and other transceiver manufacturers) simplified this connectivity by enhancing their USB device drivers to create a virtual soundcard whose input and output were connected to the transceiver's audio output and audio input respectively. In combination with the transceiver control functionality, this integral soundcard functionality means that connecting a recent Icom transceiver to your computer with an A-B USB cable provides both both, eliminating the previous need for a physical serial port, a CI-V interface, a soundcard, and a soundcard-to-transceiver interface.
The IC-7300 is the first Icom transceiver capable of not just providing a spectrum/waterfall display on its front panel, but reporting spectrum data via the CI-V bus to the transceiver control program. Commander can use this data to populate a Spectrum/Waterfall window with more functionality than available on the radio's front panel (but with a slower update rate), and the ability to overlay the callsigns of active DX stations.
In order to convey spectrum data via the CI-V bus, the virtual serial port's baud rate must be set to 115,200 baud -- several times faster than the maximum speed of a physical CI-V bus (19,200 baud). To make this work, Icom added a menu setting that can unlink the virtual serial port and CI-V interface from the backpanel CI-V port. When unlinked, the transceiver control functionality can run at 115,200 baud (and report spectrum data to the transceiver control application), while the backpanel CI-V port operates at 19,200 baud (or lower). This unlinked arrangement is depicted and somewhat documented on page 6 of this manual.