I have the following wind sensor (RT240): http://www.nrgsystems.com/FileLibrary/829de7ea934944f09798ad2830abfa65/RT240_and_RT20_Manual.pdf

It has a 8 pin connection cable and I'm trying to receive/send data from a PC by using "Full duplex pin assignment" at page 9. I want to convert RS485 to RS232 for communicating with a PC.

The point is, I'm kind of lost between protocols and interfaces in this manual. I'm trying to use RS485 interface and I have the following converter I-7520A (at page 14): http://ftp.icpdas.com/pub/cd/8000cd/napdos/7000/manual/7520.pdf

If what I'm trying to do make sense, here is how I connected the connector pins to the RS485 converter:

enter image description here

Here is how I connected the cable connector pins to the RS485 converter:

Pink (RXD-) to 5 TX-

Yellow (TXD-) to 7 RX-

Red (Control con.) to nowhere

Grey (RXD+) to 4 TX+

Green (TXD+) to 6 RX+

Blue (Analog ground) to nowhere

White (Supply Voltage - ) to 10 and they the together to + of 24V DC

Brown (Supply Voltage +) to 9 and they the together to - of 24V DC

First of all, when I power the sensor and the converter I can’t hear any sound from the sensor where I supposed to hear tickling sound normally. It looks like it doesn't power up unlike the converter. And I also don’t see any data from the PC serial port by using Windows HyperTerminal.

I’m really lost, could anyone guide me how I can use RS485 interface with this sensor and convert it to serial. There is also SDI-12 protocol in the manual which I have no idea about.

What I was trying to do was to use RS485 protocol for this sensor and receive send data in ASCII format. Could anybody give me a little help? It looks like I'm doing something fundamentally wrong here.

  • \$\begingroup\$ If you look on page 36 in the sensor manual it mentions the SDI-12 setup. You have to tie the control signal to analog ground. Then run baudrate 1200 on the terminal. \$\endgroup\$
    – Dejvid_no1
    Commented Apr 10, 2014 at 21:05
  • \$\begingroup\$ @Dejvid_no1 for SDI-12 setup my problem is how to connect the pins to the RS485 converter which I gave the link. I think you are very close to the solution. I would be very glad if you could expound on it a little bit more. thanks \$\endgroup\$
    – user16307
    Commented Apr 10, 2014 at 21:12
  • \$\begingroup\$ if i use half-duplex interface should Yellow(cable pin) go to D1+ or D1- (at the converter side)? \$\endgroup\$
    – user16307
    Commented Apr 10, 2014 at 21:23

2 Answers 2


Your sensor has some different modes of communicating:

  1. Modbus, a software protocol that can be run on the electrical standard RS485. Making manual Modbus frames on the terminal seem improbable.
  2. NMEA, combined software and electrical protocol/standard. Can be run on RS485 and others. Could probably be done on the terminal but it also requires some protocol frame building.
  3. SDI-12, combined software protocol and electrical specification. 5V logic levels. SDI-12 should be fairly easy to use with the terminal but you would need a SDI-12 to RS232 converter.
  4. Analog current/voltage. Directly measurable by an ADC or voltmeter.

You have to decide which one to use. I would test 4, first to see that the unit is in working order. Attach supply to pin7 & 8. One 1k resistor from pin 2 or 4 to ground. Measure the voltage over the resistor and see if i changes with whatever parameter was set in the factory:

"The analog measurement outputs are configured at the factory based on customer specification" //From the manual

  • \$\begingroup\$ if TXD- pin is transmitting shouldn't the receiving part be RX-? u re suggesting is to connect receiving ends to receiving ends ? \$\endgroup\$
    – user16307
    Commented Apr 10, 2014 at 20:54
  • \$\begingroup\$ Oh yeah, sorry about that. Maybe I mixed up the 4-wire vs 2-wire. Will edit. \$\endgroup\$
    – Dejvid_no1
    Commented Apr 10, 2014 at 20:57

Have you remembered your 120 ohm termination resistors, like in http://www.maximintegrated.com/app-notes/index.mvp/id/763

  • \$\begingroup\$ my cable is very short, but I might have connected the pins in wrong order. Do u agree that the sensor is using RS485 interface. I couldn't find which ASCII character to send even if I can establish the communication. \$\endgroup\$
    – user16307
    Commented Apr 10, 2014 at 20:37
  • \$\begingroup\$ 485, or maybe 422. Try the resistors, even for a short run \$\endgroup\$ Commented Apr 11, 2014 at 10:49

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