The Icom CI-V system is a serial communication protocol used to control Icom radios. It uses a simple two-wire interface (Data and Ground) that operates at TTL logic levels (5V).
The physical bus operates at standard TTL logic levels (0V for low, 5V for high). Modern USB ports operate on serial data protocols, meaning an interface must bridge the USB interface to this 5V single-wire TTL bus. 2. Core Components of a USB CI-V Interface
| | Pros | Cons | Best For | | -------------------- | ---------------------------------------------- | ------------------------------------------------------- | --------------------------------------------------------- | | Two-Transistor | Very simple, extremely low cost, uses parts on hand | May not be robust for long cables, no opto-isolation, limited drive strength | Beginners, hobbyists, confirming a proof-of-concept | | MAX232 (RS-232) | Standard CT-17 clone, uses legacy RS-232 levels, well-documented | Requires a legacy COM port or a USB-to-RS232 adapter, often needs external power | Users with older PCs that still have a real COM port | | USB Bridge IC (e.g., FT232RL) | Modern, direct USB connection, robust bus driver, can be very compact | Requires soldering of fine-pitch SMD components or buying a pre-made module | The default choice for a modern, high-quality interface | | Opto-isolated | Excellent immunity to RFI and ground loops, very professional | Most complex to build, requires more components and careful board layout | Ham radio operators with high-power stations or RFI issues |
A reliable USB-to-CI-V interface requires components that balance driver stability, noise immunity, and ease of assembly. USB-to-Serial Bridge The heart of the interface is the USB-to-UART bridge IC. icom ci v usb interface schematic top
Check the 1N4148 diodes—they are often soldered incorrectly.
When designing the printed circuit board (PCB), the layout of the top layer must prioritize RF noise mitigation and user clarity. Top-Layer Component Placement
At the hardware level, CI-V is a that operates at TTL voltage levels (0 and +5V) . It uses a single data line plus a ground, a significant difference from the separate Transmit (TX) and Receive (RX) lines found on a standard RS-232 or USB serial port. The Icom CI-V system is a serial communication
Connect all radio-side grounds together: , Pin 5 of IC 2, and Pin 5 of IC 3. Configuration and Software Setup
Look under "Ports (COM & LPT)" to identify the allocated port number (e.g., ).
If you are building one from scratch today, is highly recommended. Modern USB ports operate on serial data protocols,
If using an isolator, draw a physical keep-out zone across all layers of the PCB beneath the isolation IC. No copper traces, ground planes, or vias should cross this line. Aim for at least an 8mm clearance gap.
Building Your Own Icom CI-V to USB Interface: A Complete Schematic Guide
This is the minimalist's dream. The schematic often uses two general-purpose NPN transistors, such as or 2N3904 , along with a few resistors and diodes. It works by using one transistor to drive the bus when transmitting, and another to buffer the received data. This circuit is simple, low-cost, and effective for a single radio. However, it may not have the current sinking capability to drive a bus with multiple devices.