S-COM 6K Owner's Manual page 12

2. RECEIVER #1 (REPEATER RECEIVER) COR
Your 6K controller requires a logic signal, generated by your repeater receiver, which becomes
active when an incoming carrier is detected. This logic signal is usually called "COR" (Carrier
Operated Relay) or "COS" (Carrier Operated Switch), and is usually generated by the receiver's
noise-operated squelch circuitry. Some receivers don't have an external COR or COS connection
but do have a "channel busy" LED indicator, which works similarly.
At this point in our discussion, it is not important whether the COR signal is "low active" or "high
active" (that is, whether the signal goes to a logic low condition or a logic high condition upon carrier
detection), since the controller has sense-reversal DIP switches on the Main Board. These
switches are discussed later.
The get a better understanding of how to interface your particular COR signal to the controller, let's
examine the controller's COR input circuit. It's a general-purpose input circuit consisting of NPN
transistor Q1C and a set of three resistors. (Q1 is actually a DIP package containing four 2N3904-
type transistors. R11 is a DIP package containing seven separate 10k resistors. R12 is a SIP
package containing seven 4.7k resistors with one side common.) Resistor R11F and the resistor on
pin 7 of R12 together form a roughly three-to-one voltage divider. Q1C requires about 0.7 V to turn
on. Therefore, the COR input voltage threshold is about 3 x 0.7 V = 2.1 V. Your receiver's COR
driver must be capable of generating a signal that swings above and below this 2 V threshold as an
incoming carrier is applied and removed, or it must have an "open-collector" type driver.
The third resistor from the group mentioned above is R13 (4.7k). R13 is a "pullup" resistor, and it's
connected between the COR Input pin and the controller's +5 V supply. The purpose of this pullup
resistor is to provide input current for transistor Q1C in installations where the COR driver is an
open-collector transistor or a pair of relay contacts ("dry" switching).
Now let's get down to specific interfaces. If your receiver's COR provides a pair of relay contacts to
indicate carrier detection, then connect one contact to the controller's COR input and connect the
other contact to ground.
If your receiver's COR provides an open-collector transistor or open-drain MOSFET, then connect
your COR output directly to the controller's COR input.
Incidentally, note that some open-collector circuits will not pull down an input circuit all the way to
ground (0 V). This is normal. When OFF, these drivers are open (high impedance). When ON,
the transistor driver will pull down to a few tenths of a volt above ground. If a Darlington transistor is
used, the output is even higher above ground because of the Darlington's higher saturation voltage.
If the controller were designed with a low-threshold COR input, it is possible for such driver circuits
to keep the COR input "high" permanently. Remember that the 6K uses a voltage divider to raise
the minimum threshold of the COR Input from 0.7 V to 2.1 V. As long as the receiver's COR output
pulls down below 2 V, the COR input will operate properly.
If your receiver's COR circuit sources a voltage from a logic gate or op amp comparator, then you
may need to disconnect the COR Input's pullup resistor. Otherwise, it may interfere with the
operation of the receiver COR by trying to pull it to +5 V. You can check your installation by making
the connection between the receiver COR output and the 6K COR input, then measuring the
voltage at the COR input pin with both the receiver and controller powered up. If the voltage swings
above and below the 2 V threshold upon carrier detection, no problem exists. If the receiver COR
11
6K V1.02 03-16-91