The connection to the radio's Tip is made from this junction.
+----------------+ +----------------+ +-------+ +---------------+ | | | | | | | | | USB Port +------+ USB-to-TTL +------+ CI-V +------+ Icom Radio | | (PC) | | Serial Adapter| | Level | | (CI-V Port) | | | | (e.g., FT232R)| | Shift | | | +----------------+ +-------+--------+ +---+---+ +-------+-------+ | | | | | | | +5V, GND, TX, RX | Data, GND | Data, GND | (TTL Levels) | | | | | +-----------------------+ | | v (Shared CI-V Bus)
The most robust way is to use a logic gate, such as the (a hex buffer) or a NAND gate configured as an inverter, to actively buffer and combine the signals. A more common and simpler approach is the "two-transistor" method, which is essentially building a basic open-collector interface.
The CI-V bus is a single-wire, bi-directional serial communication system that operates at TTL voltage levels (0V to 5V).
: Solder the 1N4148 diode and 10k resistor between the TXD and RXD pins as specified in your chosen schematic to create the single-wire bus. Wire the 3.5mm Jack Connect the of the 3.5mm mono plug to the combined TXD/RXD line. Connect the to the GND pin of the USB module. icom ci v usb interface schematic top
+---------------------------------------+ | FT232RL USB UART CHIP | | | [USB]--| USB_DP TXD [01|----+ | USB_DM RXD [05|--+ | +---------------------------------------+ | | | | 1N4148 | | Diode | | +--+ | | +----- |>| -----+ | | +--+ | | | +-----------------+ | | 4.7k Ohm +--[===]--+ VCC (+5V) | +-----> [3.5mm Jack TIP] (CI-V Data) [GND]-----------------------------------> [3.5mm Jack SLEEVE] (GND) 1. The USB-to-UART Bridge
When the USB device talks, it pulls the single wire low via the diode. When the radio talks, it pulls the wire low, and the signal reaches the RXD pin. 3. Top Commercial CI-V Interfaces (Ready-Made)
Electrical signals are converted into light inside the chip and passed across an physical gap to a light sensor.
[USB Port] → [USB-to-UART bridge] → [Opto-isolation stage (TX direction)] → [Buffer/Driver] → [CI-V connector (3.5mm TRS)] ↑ [CI-V connector] → [Signal conditioning] → [Opto-isolation stage (RX direction)] → [UART RX] The connection to the radio's Tip is made from this junction
) handles the USB protocol and provides TTL-level serial signals (TX and RX).
+5V (from USB or Rig) | [ 4.7k ] | RS232 TXD o--[ 10k ]--+--| Base (PNP BC557) | Emitter o-------> To CI-V Tip (3.5mm) | Collector | GND Use code with caution.
Before looking at the schematics, it is crucial to understand how CI-V works.
The Icom Communications Interface V (CI-V) is a standard protocol used by Icom transceivers for computer-aided transceiver (CAT) control. While modern radios like the Icom IC-7300 have a built-in USB port that handles CI-V data, older or legacy models require an external interface to bridge the computer's USB port with the radio's 3.5mm "Remote" jack. 1. The CI-V Bus: Technical Overview The CI-V bus is a single-wire, bi-directional serial
Because TX and RX share the same wire, your interface must combine the computer's separate TX and RX lines into one line without causing data collisions or echo loops that confuse the software. Top Schematic Designs for a CI-V USB Interface
When reading any CI-V schematic, you will see standard symbols. Resistors have a jagged line, transistors are a circle with three leads (E, B, C), and the CI-V bus line is often just labeled "CI-V Data" or "To Radio Tip". The power bus is labeled "+5V" from the USB line. The earth or ground is a standard downward-pointing triangle symbol.
This was the standard design in the 1990s and 2000s. It converts USB to RS-232 voltages (+/- 12V), and then converts that back down to TTL for the radio.
: Download a reference schematic from reputable ham radio sites (e.g., G4ILO’s interface, K0GW’s CI-V adapter), open your preferred PCB software, and build the interface that will sit at the top of your radio desk for years to come.
