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AN21: Controlling of IP Relay using program NetCat 

Besides the RS-232 serial port, the IP Relay device contains also another two relays whose switching on/off can be controlled over Ethernet (TCP/IP). Application Note 21 describes how to control relays from the Windows command line sending right sequence using program NetCat.


Controlling of IP Relay output using command line is useful when you need switch relay on/off, for example from the another program (.bat file). Example application can be switching on the warning siren via Ethernet, when any program detected crossing of appropriate temperature, any observed parameter of gas or any similar parameter.


NetCat is simple utility, which send data included in the file to the TCP/IP connection to the specified IP adress. This utility is distributed as freeware and can be downloaded from the home page ,where you can find version for operating systems Linux, FreeBSD, NetBSD, SunOS/Solaris a MacOS X or from , where you can find version for Windows. Application can be used for direct sending commands to the appliance. Run NetCat.exe and by typing appropriate commands send the data (see below picture). List of all commands you can find in NetCat manual.   

In our application we create NVT command-file, which will switch on relay 1 on the IP Relay. This file we will save as “set_rel_1.bin“, and then send to IP address of IP Relay using NetCat and so we control outputs of Relay.

Creating of file with command

For sending command, which will switch on for example relay on the channel 1 create a file in the NetCat directory. File will only contain appropriate NVT command. Table of commands you can find in manual for appliance, but be careful commands are in binary form so you must switch your editor to HEX mode as seen in the picture below. And save it for example as “set_rel_1.bin“.

Other commands for controlling IP relay
Action NVT command
Switch on Relay 1 FF FA 2C 32 25 FF FO
Switch off Relay 1 FF FA 2C 32 15 FF FO
Switch on Relay 2 FF FA 2C 32 26 FF FO
Switch off Relay 2 FF FA 2C 32 16 FF FO
Switch on LED3 – yellow FF FA 2C 32 10 FF FO
Switch off LED3 - yellow FF FA 2C 32 20 FF FO
Switch on LED4 – red FF FA 2C 32 11 FF FO
Switch off LED4 – red FF FA 2C 32 22 FF FO
Switch on LED4 – green FF FA 2C 32 17 FF FO
Switch off LED4 – green FF FA 2C 32 27 FF FO
For sending other commands (more in table) you just create file with corresponding NVT command. And then you send it in the same way.

Compete list of commands will you find in manual for appliance…

Sending file to the appliance

Into the command line write the command:

nc parameters yy < file name

  • marks an IP address of appliance you want to send data
  • yy marks port on which appliance communicate
  • parameters you may leave this space empty or fill it with one or more parameters
    • -d will close window with the command line
    • -n NetCat will accept only digits in IP address
    • -u connection in UDP mode instead of TCP
    • -v will write basic information about connection
    • -w will set time limit for trying to make a connection and also the network inactivity


e.g. “nc –d 23 < setrel1.bin” will send command for switching on relay on channel 1 (this command is saved in file set_rel_1.bin) to the appliance with IP address over port 23.


Creating executable file

If you don’t want type every time command to the command line, you can create file, which will contain the appropriate text. For example “nc –d 99 < setrel1”. And save it as “filename.bat”. Executing this file will cause the same thing, as you write the command, which this file contain, into the command line.

Note: If NetCat will not do what you want let’s try NVT sequence in our application Hercules – TEST tab.


Related pages


Related Products and pages

  • IP Relay
    IP Relay - 1x RS-232 serial port a 2 relays with contacts on a terminal block. Remote RS-232 serial port that can be used to switch on/off or reset remote device via Ethernet.
  • I/O Controller
    I/O Controller is an Ethernet enabled unit containing 8 digital inputs, 8 digital outputs and RS-232/485 serial interface. All the interfaces are accessible over a network using the M2M protocol on TCP/IP.
  • Hercules setup
    Useful serial port terminal (RS-485 or RS-232 terminal) , UDP/IP terminal and TCP/IP Client Server terminal.
  • Other Application Notes



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Zdeněk Stupňáněk - Stupnanek@