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I have 2 different type of connectors of 3.5mm to DB9 that I use to send RS232 commands from device to a screen. The 3.5mm is attached to the device on the serial port and the DB9 to a serial cable and connected to the screen.

Now, I have 2 different devices (A and B) that can send serial commands. The connector on the left works on device A but does not work on B and the connector on the right (with pink tag) works on device B but no on A.

The 'pink' connector has a serial number, and when I Googled it, it says its a CBF Signal, and when I dig deeper it just says its a rs232 connector.

Can someone explain what are the differences between these 2 different connectors and why it works on a specific device where in the end, they are just 3.5mm to DB9 serial connectors?

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    \$\begingroup\$ Have you checked which pins are conected to which terminals on the 3.5 jack? They may well be different... Use a multimeter. \$\endgroup\$ – Solar Mike Jan 24 '18 at 8:24
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    \$\begingroup\$ there's no standard for RS232 over 3.5mm connecter, the cabple possibly have different connections between the DE9 pins and thee 3.5mm contacts. \$\endgroup\$ – Jasen Jan 24 '18 at 8:26
  • \$\begingroup\$ This needs a lot more detail to be an EE related question... \$\endgroup\$ – Wesley Lee Jan 24 '18 at 9:01

You have two cables, one works and another does not. What is similarity and what is the difference?


  • both are black cables, looking like the same;
  • both are having DB9 (male or female - can not see on the picture) at one of their ends. You know that DB9 is standard connector and is used is numerous applications and interfaces, standard and non-standard;
  • both are having 3.5 mm stereo jack at another end. Here's the same story, jacks are standard, used in a lot of applications.


  • one cable is having stickers on it, another does not. Maybe they have different purpose and made by different companies for different applications?
  • while connectors on the cable ends are standard, their shapes are slightly different. One has label "IN" on it, another does not;
  • one cable has 3.5 mm jack with 4 contacts (TRRS type), another seem to have 3 contacts (TRS type, as far as I can see on your picture).

So it is logical that adapters are not the same. How they are not the same you can find only using the multimeter as @SolarMike suggested.

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  • \$\begingroup\$ comprehensive and clear, conclusion : OP has to test them. \$\endgroup\$ – Solar Mike Jan 24 '18 at 11:04
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    \$\begingroup\$ OMG. I was really struggling this past week trying to figure the differences until you highlighted that TRRS/TRS contact. At least that gives me some lead to investigate it further. Btw, just a lil background, I wrote the software that sends the commands through their API's and they say just use an RS232 connector, I really don't have any knowledge in wiring and cable kinds of stuff but this really helps. Thanks, Doc! \$\endgroup\$ – Zaid Jan 24 '18 at 11:49
  • \$\begingroup\$ Just to add on. My colleague that has the same problem got a NULL-MODEM adapter and attached it to the DB9's end, and got it working. So another lead we can work on. \$\endgroup\$ – Zaid Jan 25 '18 at 1:31

A 3.5mm jack isn't a standard connector for serial links. The chances are the device manufacturer used one because it was all that would fit in their device. As a result, there's no standard for how the pins on the DB9 are connected to the rings on the 3.5mm plug. They are probably wired differently.

It's also possible that there's some electronics hiding in one or both DB9 plugs, acting as a protocol or voltage level converter.

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  • \$\begingroup\$ Ok. This is new to me too. I thought 3.5 is standard all this while. I guess that TRRS/TRS as what @Anonymous mentioned worth to check. At least I could go back to the manufacturer and ask about this instead asking them why some of my connectors work/dont't. Thank you too. \$\endgroup\$ – Zaid Jan 24 '18 at 11:54

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