I purchased the blue adapter in the picture as a USB/RS232 converter. It takes in the RJ12 cable from my PLC, and converts it to a USB input.

Now when I try to use an RJ12-DB9 Female and then connect that to a DB9-USB which then connects to my computer (via USBO and then try to communicate to my PLC using this method (also seen in the picture), I can't seem to get my program to work. What am I missing here? I'm sure all of my configuration settings are working, so what could be going on here?

Basically the blue adapter works with my program, but the combination of the 2 adapters in the lower part of the picture do not work and I'm wondering why. I understand there are some electronics in the blue adapter (HIN202 for example), but these chips just seem to "assist" in maintaining the RS232 communication process, rather than actually provide infrastructure for it to work.

Edit: Sorry guys....find picture attached and thank you for your comments, hopefully this pic confirms or provides better insight into my situation.


  • \$\begingroup\$ What picture? I don't see one. If you can provide a URL, I'll edit it into your question. \$\endgroup\$
    – Dave Tweed
    Aug 2, 2014 at 0:06
  • 1
    \$\begingroup\$ The wiring of the RJ12 to DB9 adapter may not be correct for your purposes. \$\endgroup\$ Aug 2, 2014 at 0:23
  • 1
    \$\begingroup\$ I'd give odds that the RJ12 to DB9 adapter is the wired reverse of what you need. Try putting a null-modem cable (and gener converter as needed) between the adapter and the USB to DB9 converter. \$\endgroup\$
    – DoxyLover
    Aug 2, 2014 at 1:11
  • \$\begingroup\$ Did the brownish adapter come with the equipment you are trying to contact? The RJ-pinout is not standardized for these purposes. I used to travel with a box full of different adapters and often had to internally rewire them for specific devices. \$\endgroup\$
    – jippie
    Aug 2, 2014 at 6:22
  • \$\begingroup\$ If the blue module is working, why don't you want to use it anymore? \$\endgroup\$
    – Ben Miller
    Aug 2, 2014 at 11:42

3 Answers 3


3 possible issues to always be aware of when using RS232 to USB converters. (in order of most common to least.)

  • Converter driver comm port number chosen matches comm number in PLC communication software.
  • Power from USB port enough to drive all devices connect.
  • Communication watchdog timer when converting from multiple protocols.

1st example: see http://bin95.com/Industrial-Training-Videos/AB-PLC-DH485-RS232-USB.htm 2nd and 3rd example: if you connect USB to computer, RS232 to RS232 to RJ485 converter box (pic box), the computer's USB port will not have enough power to power circuits in RS232 to RJ485 converter box. And/or communication during multiple conversion gets slowed down too much giving comm error on PLC side.

Solution: Get a converter cable specifically designed to convert USB to RJ485 instead of connecting two different converters together.


In my answer to your other question, I linked to the pinout required for your RJ12 cable. Check that the cable you are using matches the pinout required.

Getting USB-Serial adapters to work with the correct drivers can be really tricky. I've seen some cheap adapters with poorly written drivers not work at all. Test the USB-Serial adapter with something other than your PLC to confirm that it is working correctly.


Many PLCs use custom wiring on multipin connectors (dsub, etc). Your usb-serial must be aware of any specific changes required for that device (usually the PLC provider wants to sell you there own special converter, that will also include special drivers).

  • \$\begingroup\$ I think you hit the nail on the head....that programming cable has a few chips inside of it that deal with setting logic levels, signalling rates and latency....I imagine it's $40 (the freaking plc was only $70), for that reason....and i'm sure that's a nice healthy markup as well :) \$\endgroup\$
    – Pardon_me
    Aug 10, 2014 at 5:54
  • \$\begingroup\$ @Pardon_me It would be best to find some old computer with the connector built in, then you can rule out special configuration for the PLC adapter. If that works, then you know your USB->serial is the problem. Make sure to compare wiring diagrams from the PLC adapter to standard Dsub9, if your USB expects standard wiring, but the PLC adapter is not standard, you may need to make a little patch cable or something to get the wires in the right place. Otherwise, see if the USB driver has different emulation modes or something that can change its behavior. \$\endgroup\$ Aug 10, 2014 at 6:50

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