If the HMI is the heart of the CTR2 system, the Radio I/O modules are its arms and legs. A Radio I/O module is required for each radio in your system. Its job is to interface the Tx and Rx audio, CAT data, key and PTT output signals between the HMI and the radio.


Schematic files can be downloaded here.

Radio I/O Schematic

The Radio I/O module interfaces to the HMI using CAT5 cable via RJ45 jack J3. An RJ45 switch is required when two or more Radio I/O modules are deployed.

If you are only interfacing to one radio, use the Auxiliary Option 1 board. See the HW: Auxiliary Board post for more information.

NOTE: The Radio I/O interface is NOT Ethernet based. Never plug a Radio I/O module into an Ethernet hub, switch, or router! You must use a manual RJ45 switch or the CTR2 RJ45 Switch to route Radio I/O signals to the HMI.

The interface to the radio is provided by a 12-pin terminal strip, J2. This provides a universal interface to any radio.

For information on how to wire this module to your radio, visit the HW: Radio I/O Wiring Diagrams post.

NOTE: The Radio I/O board in the lead photo is an older v1.2 board. Current boards are v3.0 and follow the pin description below.

LINE IN – J2:11 and 12

Line In is audio from the radio’s Line Out or speaker output. It is transformer coupled and DC blocked to minimize any noise that might be picked up on this line. If the radio’s speaker output is referenced to ground either Line In pin can be connected to the radio’s ground. Jumper JP1 bypasses variable attenuator RV1. Leave this jumper out if you need to attenuate a high signal from the radio’s speaker output. Jumper JP2 inserts a 2-watt power resistor in parallel across Line In. You should install this jumper and a load resistor in R4 that matches the speaker impedance of your radio. If you are using the radio’s Line Out signal, you can leave out RV1 and R4. Just install jumper JP1 to bypass RV1.

LINE OUT – J2:9 and 10

Line Out is the audio out from CTR2 and connects to the radio’s Line In or microphone. It is also transformer coupled and DC blocked so either pin can be connected to the radio’s ground. There are no special options for this signal, just connect it to a suitable input on your radio.


Some radios may require a DSR or CTS handshake signal on their RS232 CAT port in order to operate. Pin 8 of the 12-pin terminal strip can be used to provide this signal. J1 can be strapped to output a +5 VDC or a Ground level signal. The +5 VDC signal is current limited to about 40 milliamps by R8. If you need more current, install a smaller resistor at this location. It is an SMT device.

CAT DATA – J2:4 (Gnd), J2:4 (Rx), J2:5 (Tx)

CAT data to and from the HMI arrives on J3 and is routed to SW1. SW1 is used to configure the inverters in line driver U1 to match the CAT signal levels the radio is expecting. The signal options include Straight Through (same as the Teensy 4.1 TTL I/O), Inverted (inverted signals for RS232 ports and some radios), and 2-Wire (Icom CI-V two-wire). In all cases, the CAT interface to the radio uses +5 VDC levels.

CAT data is referenced to the radio’s ground through J2:4. Rx data from the radio should be applied to J2:5 and Tx data to the radio is on J2:6.

The chart below shows which switches must to closed (on) for each option.

Straight Throughxxxxxx
2-Wire (CI-V)xxxxx


Two options are provided for keying the radio’s Key and PTT circuits.

Reed Relay

Install reed relays in K1 and K2 locations and leave JP3 and JP4 open if you want to have the greatest isolation between CTR2 and your radio. Reed relays are capable of switching up to 200 volts AC or DC at 500 milliamps. These are a great option for old tube type radios that use grid block keying. There is a 1 millisecond pickup and dropout delay on these relays.

Solid-state Output

If you are using a modern radio with low voltage keying circuits you can eliminate the reed relays and use the outputs of U1 directly. Just solder a bridge across jumpers JP3 and JP4 to bypass the relays. The keyed circuit must be positive polarity and less than 50 volts at 500 milliamps to use this option.