remote keying cdm 1250

[brin831]

hello all i have a question that i hope someone out there can answer without too much trouble ... i wanted to ask here before i went to soldering!!

I have a wired intercom station that i want to use to key a cdm1250. I have built various repeaters and am very familiar with the accy connector of the cdm's ... i would even try a vox setup but have never been too impressed with its reliability it never seems to work 100% of the time ...

so with that said here is directly from the manual of my intercom base station:

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"There are internal pull-up resistors on the CH 3 and CH 4 key outputs. Pressing the CH 3 or CH 4
talk button will result in a logic high to low transition at the corresponding key output. (Logic
high is approximately 13 VDC; Logic low is less than 2 VDC.) There are no internal pull-up
resistors on the CH 1 and CH 2 key outputs. These outputs will “float” until externally loaded."
---

my question is can i with no harm to the radio wire either key outputs of any of the channels to the cor input of the radio pin 3 if i'm not mistaken i don't have the pinout in front of me ... and use this to key the radio when the channel talk buttons are depressed ??

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second question -- i also have a xpr8300 trbo repeater used in analog mode at times ... can i use the same setup to key this repeater as well ??? because the install guide states 5v ptt input as pin 17 but i am warry of too much voltage applied to the circuit ??? i don't want to harm the radio ...

ok thanks guys

[maxkelley_kc2spy]

my question is can i with no harm to the radio wire either key outputs of any of the channels to the cor input of the radio pin 3 if i'm not mistaken i don't have the pinout in front of me ... and use this to key the radio when the channel talk buttons are depressed ??

COR input? Do you mean PTT input of the radio? And, secondly, do you want the CH 3 key output to PTT on CH3 of the CDM1250, or just whatever channel it's on?

Anywho, what I would do (and I think this would work...) Is take your PTT input to the CDM and set it to active low (meaning it will PTT with a low logic level), and have a diode on the output of pin 3 so it would look like this:

CDM Pin 3 --->|----Key output of intercom.

So, when the output of the intercom is high (no PTT), the diode will be reverse biased, and the PTT input will float high due to the internal pullup resistors in the CDM. When the intercom pulls the output low, the diode will be forward biased, and CDM's pin 3 will be pulled down to ground, with the key output of the intercom sinking that current.

I think that should work... Anyone else, feel free to correct me if I'm wrong!!

[Bill_G]

Brin - You use the PTT input on the CDM for xmit, not the COR output.

As for your intercom set, you can try it to see if it works on the CDM. I don't know about the XPR.

[brin831]

yes i mean ptt ... i was thinking how i previously wired the cdm's for a repeater setup where i took the cor from rx radio to pin 3 which is ptt to have another cdm tx.

no i don't mean ch 3 key to ch 3 of the radio i just mean whatever ch key on intercom to key the radio so it will tx on whatever ch it is on so for example ch 4 intercom key to radio allowing it to tx on whatever the active ch is ... i don't think there is a way to have the radio change channels based on accy inputs a neat thought but not necessary for what i would need it to do ...

[maxkelley_kc2spy]

Yup, the CDM has channel steering from the accessory connector for up to 16 modes on the 1250s and 1550s, and 4 modes on the 750s. On the 1250s and 1550s, it's selected via BCD (4 lines).

[brin831]

well thanks max ... i stand corrected ... never spent anytime looking into that feature as it wasn't something applicable for what we wanted ... although we are coordinated for a number of frequencies they are often times used for other things so we had one repeater pair tasked for this application and don't need to add radio keying to more than one channel.

i'll have to look into that for the future though.

i never got around to it yesterday but i'll let everyone know how it works today when i get down to the shop to wire this up.

[maxkelley_kc2spy]

Sounds good! I hope it works out!