COR Buffer Circuits
From RLC Controllers - Support
The RLC repeater controllers require some kind of signal from each receiver to indicate whether a RF signal is being received or not. That signal is used by the controller to determine whether to turn on any of the transmitters and whether to pass or mute audio from the receiver. There are two pins on each DB-9 radio port connector that can be used for this, labeled "COR" (pin 7) and "PL" (pin 2). Both inputs are identical electrically; they have a pullup resistor to 5V, a clamping diode (to allow voltages of up to at least 15 volts without damage), and a voltage detector that senses whether the voltage is "high" or "low" (more on that below). The PL input should not be connected directly to the receiver audio; it should be connected to the output of a PL decoder (either one built into the receiver, or an external one, such as Communications Specialists' DS-64) (make that a link).
The controller needs at least one of the inputs to switch between the high and low states when the receiver detects an RF signal. It doesn't matter whether it is normally high or normally low, as the controllers can be configured to work with either polarity (using command 013 on the RLC-3/4/5/Club). They default to expecting the COR to be active low (to go low when a signal is received). [add instructions for entering the polarity command with older firmware versions that won't accept DTMF when the polarity is wrong].
By default, the RLC controllers use the COR input signal but ignore the PL input signal. If both signals are available and connected to the controller, the controller can be configured to use either or both signals (using command 005 on the RLC-3/4/5/Club controllers). The manual contains a discussion of the different "receiver access modes" and when each one is most useful.
The COR (also sometimes called CAS, COS, etc) signal from most receivers can be connected directly to the controller's COR input on pin 7 of the radio port DB-9 connector. In other cases, a simple buffer is needed to ensure that the controller will be able to detect the voltage change. The voltage sensor inside the controller detects a "low" whenever the voltage at pin 7 is 0.5V or less, and "high" when the voltage is 3.5V or more. Operation in the range between 0.5V and 3.5V is undefined (it may read high or low, and may change at any time). If, after connecting the COR signal and powering up the controller, the voltage at the COR input changes from below 0.5V to at least 3.5V when the squelch opens or closes (depending on the polarity), the controller will be able to detect the change. If it does not, some kind of buffer will probably be needed.
If a buffer is needed, either of the first two options below will work 95% of the time. If they don't, more voltage measurements can be taken to determine the type of electrical circuit that is used in the receiver, and an appropriate buffer type can be chosen from there.
- This drawing might be easier to understand than the descriptions below.
- NPN transistor. The most commonly used buffer consists of an NPN transistor and a resistor. The emitter is connected to ground, the collector to the COR input on the controller, and the base is connected through a resistor (10K typical) to the COR output of the receiver. Just about any NPN transistor should work; 2n2222 and 2n3904 are common.
- If the transistor is in a TO-92 package (a small plastic cylinder with a flat side and three leads coming out of the bottom), the pinout is emitter, base, collector, with the flat side toward you and the leads pointed down.
- Voltage measurements if working correctly
- 2n7000 or VN-10K MOSFET. A MOSFET can be used exactly like an NPN transistor, except that the resistor is unnecessary; the gate (analogous to the base of an NPN) can be connected directly to the COR output of the receiver. Another advantage (besides not needing the resistor, which makes it easier to fit inside the DB-9 hood) of using a MOSFET rather than an NPN transistor is that it will work with even very weak COR outputs because it takes almost zero current to control the MOSFET. A potential disadvange of using a MOSFET is that they tend to be more static sensitive (not usually a problem once they are installed, but static handling precautions should be taken during assembly).
Measure voltage on receiver's COR pin with controller disconnected, then with it connected and powered up. Identify output type and strength.
- Open collector (sink only)
- Source/Sink (Push/Pull)
- Source only (must buffer)
- Relay (ground one side, treat like open collector)
Connecting To RX Activity LED
On Repeater-Builder@yahoogroups.com, Jim W5ZIT wrote, "I have had good luck using an opto isolator to interface a radio that I don't have the internal schematic for. The radio usually has a LED indicator for receive, and is easy to find. Place the diode section of a 4N36 isolator in series with the receive indicator LED and the transistor portion of the 4N36 will allow interface to your controller. It provides an open collector closure to the emitter when the LED on the radio is lit. Applying +5 or +12 volts to the collector will provide a positive going output on the emitter of the 4N36 when the LED is lit."