Batboard

I am trying to get pin 4 to activate a relay when receiving a mdc alert, I programmed pin 4 for external alarm and set to active low but pin 4 gets constant 12 volt even when pin 4 is programmed to null or the radio is turned off, I tried it on 3 cdm’s and on 1 gm300 with older firmware and all with same results, can any one tell me what I am doing wrong?

Active low is the only setting that the CDM will recognize as valid for this function. So your external relay/device should have one side tied to +12, with a diode across the coil observing the correct polarity if it's a relay, and the other side connected to pin 4. Is that how you've wired it?

i am trying ro gert 12 volts on pin 4 for 1000 msc. if it receves a mdc alert then drop to 0 or 5 volts when it times out, i didnt do any wireing yet I just messuerd pin 4 and it has 12volts if the power cable is pluged in to the radio no mater what pin 4 is programed as or if the radio is on or off

Pin 4 in a CDM is connected to constant 12v through a 4.7K resistor, when activated it is pulled to ground by a transistor. So it is normal to measure 12v on it even if the radio is off.

I know Maxtracs are the opposite.. they have a resistor to ground and the transistor switches 12v, I don't recall which way the GM300s are, I believe they're the same as the CDMs.

I did it on the spectra vip pin 54 and 58 it works like a champ I just need a radio with cor any ideas

Try this link for info on a relay that we have used for numerous functions such as you need. It has provision for either a + or - 1.2 mA trigger. We use it on Pin 4 setups for Alarm Out to decoders and CAS/COR. It's great for isolating the radio I/O from whatever else you're connecting it to.

http://www.elkproducts.com/_webapp_2981 ... tive_Relay

Pin 4 is designed to drive a relay, it shows how to hook it up in CDM the install manual. Pretty much what Al said, pin 4 goes to one side of the relay coil, with the other side of the coil connected to +12v, and a spike diode is a good idea (1N4007 across the coil with the striped end towards +12v)

Are you having trouble getting pin 4 to active with MDC?

My trouble is pin 4 is all ways active even if radio is powered off or pin 4 programmed to noll

As Jack explained, pin 4 in a CDM is held high (12v) by a resistor. So you will always see 12v present. Pin 4 is an open collector output that provides a ground for your relay. You might not see it go to ground with a voltmeter without a good load because of the high impedance of the voltmeter. You may be able to see it go low with the diode checker in your VOM however. Or, you can connect a 100 ohm resistor from pin 4 to ground and watch the voltage with your VOM. Or, you can hook up a relay to 12v on one side and pin 4 on the other so you actually see the relay actuate.

911-EMT wrote:My trouble is pin 4 is all ways active even if radio is powered off or pin 4 programmed to noll

Yes, that's how the CDM treats pin 4, which in your case should work out fine. One side of your coil goes to +12, the other side to pin 4...program pin 4 active low and when it receives the MDC it'll go low, pulling the voltage through the coil & activating your relay. This has the added benefit of not tripping the relay in case someone turns the radio off, as the pin remains high.