When the phototransistor conducts, it pulls the CI-V bus down to ground, replicating the serial start/stop bits. 2. Receive Path (Radio to PC)
Connect a between the TXD line and the RXD line. The cathode (the side with the stripe) must face the TXD pin of the serial chip.
This comprehensive guide details how to build a high-performance, electrically isolated USB CI-V interface. It covers everything from schematic design to physical PCB layout configuration. 1. Understanding the Icom CI-V Protocol
9600 or 19200 (Ensure this matches the setting inside your radio's menu) Data Bits: 8 Parity: None Stop Bits: 1 Handshaking: None / Flow Control Off
A very low-cost chip frequently found on Arduino boards, though driver installation can sometimes require manual intervention on older operating systems. TX/RX Combining Network icom ci v usb interface schematic top
USB Type-B USB-to-UART Level Shifter CI-V (to radio) ───────── ────────────── ───────────── ───────────── VBUS ────────────────► VCC (5V) │ D+ ──────────────────► USBD+ (FTDI/CP2102) D- ──────────────────► USBD- │ GND ──────────────────► GND │ TXD ──────────┬──────────► Level Shifter (e.g., 2N7000 or MAX232) RXD ◄─────────┼──────────► │ │ └─── 10k pull-up to 5V ──┐ │ ┌────┴────┐ │ 2N7000 │ │ MOSFET │ └────┬────┘ │ CI-V Data ────────────┘ (Open collector) (3.5mm jack tip) GND ─────────────────── (sleeve)
. These chips handle all USB protocol tasks and provide TTL-level Transmit (TX) and Receive (RX) signals.
A DC-DC converter to provide isolated 5V power to the radio-side circuitry, ensuring total electrical separation. 3. Detailed Circuit Schematic
: The CI-V (Computer Interface 5) protocol uses a bi-directional, single-wire TTL signaling system where a single "DATA" line is held high until a device pulls it low to initiate communication. USB-to-Serial Conversion When the phototransistor conducts, it pulls the CI-V
While a direct, non-isolated connection will work, it risks creating ground loops. Ground loops introduce AC hum, digital switching noise into your audio path, and—in worst-case scenarios—allow Stray RF to damage your computer or transceiver. Optocouplers physically separate the computer's electrical ground from the radio's electrical ground using light signals. 3. The Top CI-V USB Interface Schematics
Integrating an Icom transceiver with a computer opens up a world of automated logging, digital modes, and software-defined control. Icom uses a proprietary control system called the Computer Interface 5 (CI-V). While commercial USB-to-CI-V cables are widely available, building your own interface is a rewarding, low-cost project that provides excellent isolation and protection for your radio.
Option B: The High-Performance Optoisolated Circuit (Top Recommendation)
| Symptom | Likely area (top-level block) | |-----------------------------|----------------------------------------------| | No control, but radio responds to CI-V commands from another device | USB-to-UART bridge (check drivers, TX/RX LEDs) | | Erratic control, especially on transmit | Lack of opto-isolation or ground loop | | Computer sees COM port, but no PTT or frequency readout | TX opto (IC2) or buffer transistor dead | | Stuck data (bus always low) | Open collector transistor shorted, or missing pull-up on CI-V line | | Interface works only at low baud rates | Optocoupler too slow (replace 4N35 with 6N137) | The cathode (the side with the stripe) must
The FTDI-based schematic requires FTDI VCP Drivers to be installed on the PC. 5. Troubleshooting the CI-V Interface No Communication: Swap TX and RX wires.
Before looking at the schematics, it is crucial to understand how CI-V works.
By building this interface, you shield your expensive transceiver from harmful electrical surges while securing a flawless, lag-free data pipeline for automated digital operation.
Building a custom, top-tier bridges the gap between older transceivers and modern logging, contesting, and digital mode software like Ham Radio Deluxe or WSJT-X. This guide details the electrical engineering behind the CI-V bus and provides a high-reliability, open-collector schematic using widely available components. Understanding the CI-V Bus Architecture