The Icom CI-V USB interface is not a simple cable – it is a bidirectional, inverted, open-collector level converter. The top schematic presented here (USB serial chip → NPN inverter/open-collector → CI-V bus with pull-up) has become a de facto standard because:
Before building or purchasing an interface, compare the vendor’s schematic against this “top” design. If it lacks the open-collector NPN inverter stage on the transmit line (or uses a simple transistor without correct biasing), it will likely fail with Icom’s protocol.
For further reading, search for “Icom CI-V specification document” (Icom publishes the electrical spec) and open-source designs like “USB-CI-V by YO3FVR” or “W6IPA CI-V interface schematic.”
The Icom CI-V USB interface serves as a bridge between a computer's USB port and the single-wire, bidirectional CI-V (Computer Interface 5) bus used by Icom transceivers for rig control. Core Schematic Components
A modern USB-to-CI-V interface typically revolves around a dedicated USB-to-Serial converter chip, such as the FTDI FT232RL Go to product viewer dialog for this item. .
USB Interface: A USB Type-B or Micro-USB jack connects to the host computer. USB-to-Serial Chip : The (or similar chips like the
) handles the USB protocol and provides TTL-level serial signals (TX and RX).
Signal Conversion (Combining TX/RX): Because the CI-V bus is a single-wire system, the unidirectional TX and RX lines from the chip must be combined. This is often done using:
Diodes/Resistors: A simple passive circuit using a switching diode (like a 1N4148) and a pull-up resistor to combine the lines into one. icom ci v usb interface schematic top
Logic Buffers: Some designs use a hex buffer (e.g., 7417 or 4049) to isolate and combine signals for better stability.
Radio Connector: A 3.5mm mono phone plug is the standard physical connection to the radio's "REMOTE" jack.
RFI Suppression: Ferrite beads and small bypass capacitors (e.g., 0.01 µF) are often added to the data lines to prevent radio frequency interference from disrupting the connection. Typical Circuit Layout Key Design Considerations
Isolation: High-end designs may include opto-isolators to prevent ground loops between the computer and the radio.
Power: The interface is typically powered directly from the computer's 5V USB bus, eliminating the need for an external power supply.
Drivers: If building your own, ensure you install the appropriate Virtual COM Port (VCP) drivers from the chip manufacturer (e.g., FTDI) to allow software like Ham Radio Deluxe or FLDigi to communicate with the rig. Explore USB with this USB to CI-V Interface - N5DUX
The Icom CI-V (Computer Interface 5) USB interface schematic typically centers around a USB-to-Serial converter IC, such as the FTDI FT232RL Go to product viewer dialog for this item. Go to product viewer dialog for this item.
, which bridges the computer's USB port to the radio's single-wire TTL bus. Core Schematic Components USB-to-Serial Converter: Modern designs often use the or Go to product viewer dialog for this item. The Icom CI-V USB interface is not a
. These chips handle all USB protocol tasks and provide TTL-level Transmit (TX) and Receive (RX) signals.
Single-Wire Bus Logic: The CI-V protocol uses a bi-directional, single-wire system where the TX and RX lines from the converter are tied together. Level Conversion
: Because the radio operates on a shared "open collector" bus, the schematic usually includes a diode (like a ) or a buffer (like a
) to prevent the TX line from interfering with the RX line when the radio is sending data.
Pull-up Resistor: A resistor (often around 4.7kΩ to 10kΩ) pulls the DATA line high when idle. Isolation & Protection:
Ferrite Beads and Capacitors: Used on the USB power line and radio data lines to block RF interference and noise. Isolation Transformers
: Some advanced homebrew designs include isolation transformers to prevent ground loops between the computer and the radio.
Push-To-Talk (PTT): Many schematics add a sub-circuit using the RTS (Ready to Send) or DTR (Data Terminal Ready) line from the serial converter to trigger a transistor (e.g., ) for PTT or CW keying. Connection Layout Before building or purchasing an interface, compare the
USB Side: Connects to the PC; the converter chip is powered by the USB 5V line. Radio Side: Uses a 3.5mm mono phone plug. Tip: Data signal (bi-directional TTL). Sleeve: Ground reference. Icom IC7300 A to Z #27 CI-V Settings
After building, set virtual COM port to:
Set the radio’s CI-V address (default 0x5E for IC-7000, 0x76 for IC-7300, etc.) in your software (e.g., WSJT-X, FLRig, HRD).
Before etching PCB or soldering on a protoboard, verify that your icom ci v usb interface schematic top includes:
Radio → PC (Receive path):
Many cheap interfaces omit the inversion on TxD and just swap RX/TX – this fails with Icom because the protocol expects the idle state inversion.
For DIY builders, the classic Q1 (NPN) + two resistors design is the minimalist “top” schematic. No PNP, no active pull-up. Just:
USB TTL TxD ---- 10k ----- Base of NPN
Emitter - GND
Collector ---- 1N4148 ---- CI-V data line
|
+--- 4.7k pull-up to 5V
CI-V data line ---------------------------------------------- USB TTL RxD (with internal pull-up enabled)
(Note: Some FTDI chips allow enabling internal pull-up on RxD, eliminating external 4.7k)
The term "top" could also refer to the top side of the PCB or top layer routing in an EDA tool like KiCad or Eagle. For a successful layout: