The CTR2 RJ45 Switch automatically routes Radio I/O signals from one of 16 radios to the HMI. To do this, a simple switch matrix was developed using 16 banks of four DPDT miniature relays. When the HMI sends the switch an address one bank of relays is energized. They switch the 8 Radio I/O lines from the selected RJ45 port to the HMI Radio I/O port. An LED on the front of the board, and the green LED on the RJ45 port light to indicate which port is active.

Information on building this board can be found here.

In keeping with the CTR2 design philosophy of build only what you need, the 16 ports were divided into four separate boards. The first board is the master control board. It is always required and always routes ports 1 through 4. Up to three expansion boards can be added, with each board routing 4 ports. The port range for each expansion board is determined by the expansion connector (J4, J5, or J6) their 10-pin Expansion connector (J3) is connected to on the master board. The Radio I/O bus from the HMI is daisy-chained to all switch boards using a multi-tap 10-conductor ribbon cable.


Schematic files can be downloaded here.

The schematic for one switch board is shown below. The same PCB is used for all boards. The only difference between a master board and an expansion board is the components installed.

RJ45 Switch Schematic


While the schematic is busy, it’s pretty simple.

The RJ45 Switch Control ribbon cable from the HMI connects to J1 on the master control board. This cable carries addressing and control signals for the selection logic. Address bits 0 to 3 connect to Data_1 through Data4 on IC3, a CD4514 four-to-sixteen decoder. When the address data nibble has been asserted the Strobe line is brought high then low. This causes the decoder to decode the address and latch one of sixteen outputs. The output pins are not enabled until the INHIBIT line is brought low. At this time, the selected output goes HIGH. This signal is applied to the input of one of the ULN2803A Darlington drivers. These drivers are inverters so the HIGH input causes its output to go low. This signal pulls the relay coils of four relays low.

The Radio I/O bus from the HMI connects to J2 via a multi-tapped 10-conductor ribbon cable. The number of taps on this cable depend on the number of expansion boards installed. On a 4-port switch with no expansion boards a single ribbon cable connects the HMI to the RJ45 Master Switch. Eight signal wires from J2 are distributed to the N/O contacts of all 16 relays. The coils for each group of four relays are tied together and controlled by one output of the ULN2803A Darlington drivers. When that output goes low all four relays pickup connect their N/O contacts to the COM contact, completing 8 circuits between the selected RJ45 port and the Radio I/O bus. One, and only one relay can be active.

On the master switch board, the first four drivers on IC2 control the four relay banks. The other half of IC2 and all do the drivers on IC3 are connected to expansion ports J4, J5, and J6 respectfully. These connectors are only installed on the master switch board. The Port Expansion connector (J3) is only installed on the expansion boards. It connects the four relay banks on the expansion board to one of the expansion port connectors on the master. The allows simple cable connections between boards to map each boards relay banks to a specific address range.

U1 is a 3.3 volt LDO regulator that supplies 3.3 volts to CD4514 so it matches the logic levels on the Teensy board on the HMI.

A simple monitor circuit is provided to allow you to monitor receive audio from one of the radios on each switch board. This is done with four 3-pin headers on each board. These headers allow you to connect receive audio from one of the four radio I/O ports to isolation transformer T1. The output of T1 can then be connected to an external amplifier. This give you the ability to select one of your radios as a monitor receiver.

Pins 1 & 2 on the each 3-pin header connect to the “always on” audio on the Radio I/O‘s Line Input port (which is connected to the radio’s speaker or Line Out port). Pins 2 & 3 on the 3-pin header connect to the “off-line” Line Input audio. Using the “always on” connection keeps the audio from that receiver connected to the monitor amp even when you are using that radio. When the “off-line” pins are used the audio to the monitor amp is cut when the radio is active.

T1 and the associated header pins are optional and don’t need to be installed if you don’t plan on using this feature.


The graphic below shows the ribbon conductor cabling required to create a 16-port switch.

NOTE: The red conductor shown below is a multi-tap ribbon cable with ribbon connectors installed at five locations on the cable.

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