I'm looking to build my own mini-plug to DB9 adapter. I've yet to see a CM-232 schematic for the pin-out for the 3.5mm 4 conductor mini-plug.

Seems as if there's a bit more to it than a DB9 connector to 4 pin mini-plug. When I ohm out all the pins on the my own CM-232 DB9 to the mini-plug, I only get pin 5 (Signal Ground) on the DB9 to ohm to last conductor of the mini-plug. Pin 3 (Tx) on the DB9 does not ohm out to any conductor on the mini-plug.

I'm guessing there is some sort of "NO" Relay in the head of the CM-232 that receives voltage from the RP-6 and then then connects the circuit.

Anyone have any experience with this?

Thanks -

Does this from the CM-232 description on the RTI website offer any help?


They don't define "tide high." That means it's connected so there's always a voltage on it, but I don't know where they'd get that voltage. I guess it would be from a part of the mini plug.

What do you mean "to ohm out?" I think you mean to measure resistance, but when you say that, I don't know if you mean you are reading a resistance or zero resistance. I'd say it this way, and please see if this accords with what you find:

I get zero ohms between pin 5 (Signal Ground) and the sleeve of the mini-plug, and XXX ohms (you tell us) between YYY and ZZZ. All other pins are open (i.e. infinite resistance from each one to each one).

do you get zero ohms between TXD and one of the connections on the mini plug? Which one? I guess on a four conductor, starting at the tip, you'd have tip, ring one, ring two, sleeve. Sleeve is usually ground.

If the ohmmeter shows infinite resistance, there's no connection. If it shows zero resistance, there's a straight connection. If it shows a resistance between zero and infinite, that's important to spell out.

Why don't you cut one open and see if they tied any pins together, for instance 7 and 8?

Any reason you want to recreate the wheel that works just fine??

In the end of the CM-232 cable(232 side) there is a small foil wrapped circuit board molded into an epoxy like shape, that circuit board probably has some resistors and such to change voltage from 12 to 5 for correct 232 operation, I figure by the time you get one apart, figure it out and get parts to build one you could have bought a handful and had it working several times over

there is indeed a circuit board inside the CM-232. it comprises of much more than just a few resistors. Metering the pins will be of no help to you i am afraid. to DIY this one will cost you more than buying one from your dist.

Tried it and I agree

Appreciate all the great feedback. Feels as if all the Admirals showed up for this response. This was a post that just came out of some curiosity on a project I'm working on. For what a RTI system gets billed out for, paying for the CM-232 is a non-issue. However, there's still a little tinkering blood left in me and I couldn't resist posting in regards to my assumptions.

On this current project I'm using 232 to control a Receiver which has a pre-wired Cat-5 run to a second floor, about 30 to 40 feet away from the RP-6. Instead of just creating an extended serial cable to a CM-232, thought I'd just go direct and create a 4 conductor mini-plug to DB9. Looks as if the answer lies in just making my extended DB9 cable and attach to the CM-232. Not a problem.

Thanks again and best to you all.

Hi guys...
Easy answer. It is not only level convertor, there is also some stupid electronic. When I tried to connect there some circuit only to convert TTL output levels from RP6 to RS232 standards, it gives me some more "0" characters on output, on both start and end of the command ....

So with the XP8 having RJ45 for RS232, What couplers/converters are you using at the component end?

It comes with RJ45 to DB9 adaptors.

Sweet! Thanks.