While going through some (really) old computer parts I came across a very interesting cable: a female RS232 (9-pin serial) to 3.5mm stereo jack.

Any ideas what this cable might have been used for? (it's not a makeshift DIY cable)

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I remember having an old Fritz ISA > ISDN card using a RS232 to RJ45 connection.

  • Also used for Monarch Pathfinder label printers, for uploading files/firmware to the Rom-Dos memory segments – user82308 May 21 '11 at 12:51

Some devices (such as graphing calculators) use 3.5mm jacks to transmit data. They use similar signals to normal serial ports, but use a different connector.

  • 1
    close, i eventually found the corresponding device, it's a data logger for a cold storage, logging temperatures from various sensors – Molly7244 Dec 13 '09 at 16:27
  • That is the type of cable that iogear ships with their KVM (and maybe more) to upgrade the firmware. – gavaletz Jan 29 '10 at 3:44
  • Currently still new devices which uses these type of cables are sold today: conrad.com/ce/en/product/101367/Voltcraft-CM-100-CO2-Meter – Ruut May 28 '14 at 12:21

Here's one use I found online: RapidRun DB9 to 3.5mm Adapter

Cable image


Eliminate the need to run all new cables for RS-232 control of your Audio/Video system. RapidRun's new RS-232 Adapter gives you the flexibility to use existing 3.5mm stereo audio cables or DB9 cables to run 3 conductor RS-232 control signals between your control unit and your video display. This product can be used with all of the DB9 Break-Away Wall Plates and Flying Leads designed for the RapidRun system. RapidRun RS-232 Adapter Features: Supports transmission of serial control signals including RS-232. Fully molded DB9 and 3.5mm connectors provide excellent strain relief. Ideal for use with computers, projectors, and any control unit/display requiring 3 conductor RS-232 serial control. Note: Not all serial control methods are supported by this adapter. This product is intended only for use with 3-wire RS-232 control cables with common ground (pin 5), transmit data (pin 3), and receive data (pin 2). For the exact pinout of your audio/video equipment please contact the manufacturer.

  • that's exactly the cable ... however, i'm still puzzled how it came into my posession, as i certainly don't have any such 'control units' or RapidRun systems :) – Molly7244 Dec 8 '09 at 1:15

They are typically used as a pinout for digital cameras.

  • quite possible, although i only have 2 digi cams (not much of a photographer myself, and older 4 MP Samsung and a 10 MP Fuji, and that cable is probably older than the Samsung. it's not an accessory that i can find any reference in the manuals for either. – Molly7244 Dec 8 '09 at 1:20

There are some devices (like say a RAID box I have at work) that use a 3.5mm jack instead of a standard 9 pin port to access their built-in console.


I found a cable like this too, and finally figured it out that it came with my old late 1990s Olympus D-360L digital camera. It was used to transfer pictures to the computer (before card readers became common) and as I recall it would use a lot of battery transferring the pictures.


since you only really need three lines to do serial communication (and in this case we are not using hardware flow control, which would require two extra lines), a DE-9 to stereo jack is all you need for serial communication with anything with an embedded microprocessor.

The two cases I've used this are a serial programming cable to a TI cable modem, so you could get boot messages and command line access for firmware upgrade purposes; and a OneTouch UltraMini Glucose meter. Here the parent company will sell you an overpriced cable just so you can download your timestamped historic testing data into their "free" software app.

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