Brace with me as I am new to electronics, but am also eager to learn. I have a sensor which has 4 un-terminated wires:

1) 12V to 24V

2) RS-232 TxD

3) RS-232 RxD

4) Ground(and serial connector return)

I'm trying to get the sensor and my computer to talk but have faced nothing but failure. I have an RS-232 to TTL converter followed by an RS-232 to USB adapter. Here's where my real question starts. I understand that TTL has VCC, Tx, Rx and GND pins. Can my sensor talk to my computer using only the Tx and Rx wires? From my understanding, the two other wires need to be connected to an external battery since it draws so much power...but that leaves VCC and GND empty on my converter.

I've come to realize this morning that my RS-232 to USB adapter was out of date and not supported on my current version of windows. I have a new one on its way but still feel something is not quite right with my setup. Any help is greatly appreciated.

Thank you!

EDIT: Some extra information about my sensor: Digital output is RS-232 as ASCII chars, runs on 12-24 VDC(30mA@12V), configuration is 115200,8,N,1. I've also come to realize I probably don't need my TTL converter. How then would I communicate with only Rx and Tx? (Also, I know that Tx must go to Rx and vice-versa)

  • 1
    \$\begingroup\$ Some additional info on your sensor would help (datasheet ?), but what makes you think that you need the RS-232 to TTL converter (does your USB adapter have TTL levels on its 'RS-232' side?)? Are you sure you have the Tx and Rx directions properly figured out (sometimes the labeling can be a little obscure) ? \$\endgroup\$
    – brhans
    Jul 18, 2017 at 15:24
  • \$\begingroup\$ You MUST connect Ground as well as the TX and RX lines between the sensor and the RS-232<->USB adaptor. The power supply for the sensor must connect to the 12-24V connection, and to the sensor Ground. \$\endgroup\$ Jul 18, 2017 at 15:37
  • \$\begingroup\$ @PeterBennett Thank you. So I need minimum Tx, Rx, and ground. However, if my ground goes to the adapter, how will I complete my circuit for the battery? I would only be left with one wire (12-24 VDC) \$\endgroup\$
    – Aseg
    Jul 18, 2017 at 15:45
  • \$\begingroup\$ @brhans thank you for making me realize TTL had no role in this! :) \$\endgroup\$
    – Aseg
    Jul 18, 2017 at 15:46
  • \$\begingroup\$ You must connect the sensor's Ground to both its power supply and to the RS-232<->USB adaptor. \$\endgroup\$ Jul 18, 2017 at 16:06

2 Answers 2



simulate this circuit – Schematic created using CircuitLab

Figure 1. Wiring diagram.

It sounds as though you have an industrial sensor as 24 V is standard for these. You need to supply 24 V DC between the Vcc and GND terminals. You also need to use screened data cable back to your RS232 port.

If your sensor sends out a continuous stream of data then you can monitor this with a serial terminal emulator such as PuTTY to see if you can make any sense of the data.

  • \$\begingroup\$ That's exactly the case. Thanks a lot for your help :) \$\endgroup\$
    – Aseg
    Jul 18, 2017 at 17:04

There are ways of "stealing" power from an RS232 port, but 30mA is pushing it, and you may not even get 12V. So it's likely you'll need a battery or other power supply between 12V and Ground to power the sensor.

You can make an RS232 connection with just 3 wires - Rx Tx and Ground. You need a cable with the Rx and Tx swapped, and the Ground wire connected at each end.

If the two ends are reluctant to talk, Google "rs232 three wire handshake looping". You need to loop back some output pins of the RS232 port to fool the port into thinking that it is fully connected to something at the other end.


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