The functional diagram above shows the major components of the CTR2 system.

The core of the system is the HMI. It contains the brains of the system. The system is designed such that additional option boards stack on the HMI board using M3 25mm+6mm male/female standoffs. Each board added to the stack adds a little over an inch (26.6mm) to the overall height. Boards are slightly less then 4″ square. Power, USB connections, radio I/O, antenna switch wiring, microphone, headphone, key paddles, a remote PTT switch, and the remote display all connect to this stack. That’s A LOT of cables! The remote display allows the stack to be placed in a convenient (even hidden) location for optimal cable routing. Boards can be stacked in any order or laid out on a flat surface. The only consideration is the length of the 10-conductor ribbon cables used to interconnect the boards.

The stack above is my current system. It is capable of controlling 12 radios and 8 antennas. Four additional radio’s can be added by adding another RJ45 switch expansion board and replacing the side panels to allow for the additional board.


The HMI board is the core of the system. It typically mounts on the bottom of the stack. This board contains the Teensy 4.1 development board, Teensy Rev D Audio Adapter board, and the ESP8266-01 WiFi module. All boards plug into .1″ pin header sockets so they can easily be removed if needed. This board comes completely assembled (minus the Teensy and ESP boards) and tested.

Additional details on the HMI board can be found in the HW: HMI Board blog post.


The HMI board is controlled using a Nextion 3.5″ or 5″ Enhanced color touchscreen display and a rotary encoder. Both are mounted in a wedge shaped acrylic enclosure which connects to the HMI using a single short CAT5 cable. (Use CAT6 cable if your display will be more than 6 foot away from the HMI.) This arrangement allows the display to be placed at any convenient location in your station. I keep mine near my key and keyboard.

The display requires a display driver board to convert RS232 to 3.3V logic levels. For more information on the display hardware, visit the HW: Remote Display Driver Board blog post.

An alternative to using a hardware display can be found here.


A Radio I/O module is required to interface the HMI to each radio. If you are just starting out and only need to control one radio, order the Auxiliary-Option 1 board with the integrated Radio I/O circuitry. A photo of this board is shown below. This configuration will allow you to control a single radio without the need of any other hardware. Order the Auxiliary-Option 2 board if you plan on connecting to 2 or more radios. This option has just J3 and J4 installed and acts as a 10-pin ribbon cable to RJ45 adapter.

More information on this board can be found on the HW: Auxiliary Board blog post.


Each radio in the system requires a Radio I/O module. This module routes the radio’s transmit and receive audio (transformer isolated), CAT, Key, and PTT signals to the stack using common CAT5 cable. The Radio I/O module includes a configurable level converter for the CAT data signals. It can interface CAT signals at 5 volt normal and inverted levels and supports the Icom CI-V 2-wire interface.

The Auxiliary-Option 1 board contains a Radio I/O circuit and is a good option if you’re just starting with CTR2. To control more than one radio you should order Auxiliary-Option 2 (or disable the onboard Radio I/O circuit on the the Auxiliary-Option 1 board and install J3, an RJ45 jack). Use a CAT5 cable to connect the RJ45 jack on the Auxiliary board to the common port of a manual RJ45 switch then connect all of your Radio I/O modules to the switched ports on that switch, again using CAT5 cables. This allows you to select a radio with just a touch of a button.

In the near future you will be able to further automate your station by replacing the Auxiliary board with the CTR2 RJ45 Switch. This switch will automatically switch the Radio I/O signals of the selected radio to the HMI. This board will be available after QEX publishes the CTR2 Automation Options article in the Nov/Dec 2021 issue.

NOTE: The Radio I/O is not Ethernet based – Ethernet switches, hubs, and router CANNOT be used to select a Radio I/O module.

A photo of an external Radio I/O module is shown in the photo below. Wiring schematics for various radios are published under the Schematics category.

For more information about the Radio I/O module check out the HW: Radio I/O Board blog post.

The remainder of the stack is taken up by the CTR2 RJ45 Switch and two antenna switch controller boards. These boards are required to completely automate the system as shown in the lead photo. An article covering their design and construction will be published in the November/December 2021 issue of QEX. I’ll post additional information here about these interfaces at that time.

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