The CTR2-Mini I/O Multiplexer is a small, purpose built four-port switch for the CTR2-Mini ecosystem that automatically selects the one of four ports based on the Radio Port selected in the Mini. Two common ports are provided on the front panel. The first serves as the connection to the Mini’s RJ45 Radio I/O jack and the second is used to daisy-chain to another I/O Multiplexer to expand the number of ports that can be routed. These ports are wired in parallel so it makes no difference which port you use. Up to three additional multiplexers can be added to the system bringing the total number of radio ports under the Mini’s control to 16.

The multiplexer(s) are powered by the Radio I/O jack on the Mini so no additional power connections are required. There are no user controls on the multiplexer so it can be located in a convenient location in your station. The four status LED show which port is selected. If the multiplexer is not within sight range you can verify it is working correctly by changing the frequency on the Mini. The frequency of the selected radio should move with the Mini.

The multiplexer is controlled by a Seeed Studio Xiao SAMD21 development board.

The front panel is shown in the lead photo above. The back panel and the PCB are shown below.

I/O Multiplexer back panel

I/O Multiplexer PCB

What It Does

The I/O Multiplexer replaces the manual RJ45 switch that is required if you want to use your Mini to control two or more radios. It provides automatic radio I/O routing when you select a Radio Port in the Mini. In addition, it provides four open collector outputs that follow the selected port that can be used to control a remote antenna switch. The diagrams below show the basic Mini configuration for four radios using a manual RJ45 switch and the same functionality with antenna control using the I/O Multiplexer.

Multiple Radios using a 4-port manual RJ45 switch
Multiple radios using the I/O Multiplexer

Up to four I/O Multiplexers can be daisy-chained using the Expansion jack on the front panel to route up to 16 radios and their antennas. Each radio requires a CTR2-Mini Radio I/O module for basic radio control. The multiplexer also supports the new CTR2-Mini Audio Controller and its companion CTR2-Mini Audio I/O modules if you want to route Line-In and Line-Out audio to your radios too.

NOTE: The multiplexer’s antenna switch output is open-collector, meaning the selected antenna port goes to ground when it’s selected. If your antenna switch requires +12VDC to be asserted to activate its relays you’ll need to add the optional CTR2-Mini ASD antenna switch driver module. This module is supplied with the I/O Multiplexer when you order the assembled version. The printed circuit board is supplied if you order the I/O Multiplexer PCB kit. The antenna switch driver requires a separate +12 VDC supply to power your antenna switch.

How It Works

The multiplexer uses a Xiao SAMD21 development board that monitors the CAT Transmit Data (CAT TD) line from the Mini to the radio’s I/O module. When you select a Radio Port in the Mini the normal CAT data stream is stopped and the Mini resets its CAT serial port’s baud rate down to 300 baud. It then sends a port select command. After a second or two the Mini sets up its CAT port for the selected radio and baud rate and initializes it.

The multiplexer is also set to 300 baud and is listening to the CAT TD line. It ignores the normal CAT data stream because it’s running at a much higher baud rate. When it sees a valid port select command it switches ports.

Options, Options, Options

The I/O Multiplexer is just one of the new products I’ve been working on. I’ve also developed a new CTR2-Mini Audio Controller. This unit brings audio management to the CTR2-Mini ecosystem allowing you to share two microphones and a set of headphones with every radio in your station. They also have an audio port that you can connect to a PC sound card that allows you to connect your PC to every radio in your shack. The diagrams below show these configurations.

More Information

You can order the PCB kit and build the multiplexer yourself using the BOM here. As of February 2023 the cost of all the parts with shipping to the US is around $71, plus the cost of the PCB kit. The PCB kit comes with the multiplexer PCB, the front panel, the rear panel, and the mounting hardware for the PCB. I can also provide assembled and tested units for an additional cost.

The optional CTR2-Mini ASD (Antenna Switch Driver) is included when you purchase either the assembled I/O Multiplexer or the multiplexer PCB kit. This module converts the open-collector outputs from the multiplexer to +12VDC active outputs to drive antenna switches like the DX Engineering RR8B-HP that require +12VDC to pull in their antenna relays. The open collector outputs work fine with the antenna switches such as the modular antenna switch kit from

You can find the schematic and the BOM here and the operation and construction manual here.

Contact me if you have any question about this unit,

73, Lynn, KU7Q