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Connecting a RS485 cable

Last Updated: Jul 05, 2017 03:17PM CEST

Recommendations when performing a RS485 installation

  • Use a different color for each wire (one for the positive, one for the negative and one for the ground) and include them in the same bus. This prevents polarity change errors.
  • Always leave some extra cable on each connection. It makes it much easier to fix wires that incorrectly connected. 
  • Shielded twisted pair is always recommended, as it minimizes interferences.
  • It's useful to review the installation with a multimeter to ensure there are no short-circuits or loose wires.


Taking a reading

We need:
  • Modbus reading program (qModMaster) or DEXGate2 "Read & Write registers" feature
  • Slave Modbus device
  • Modbus direction of the slave (from 1 to 247)
  • Initial register (from 0 to 65535) which is the number of the register we want to start
  • ​Number of registers to obtain. That's it the numbers of all successive registers we want to obtain
  • Type of registers to obtain
  • ​In Modbus RTU: Speed, parity and stop bits
  • In Modbus TCP address: IP address and port

Detecting errors

The questions we have to do in order to detect possible errors, are the following:
  1. All devices are configured to the same speed, parity and stop bits? If not, configure all devices with the same communication parameters.
  2. From a particular node are not working the rest of devices? In afirmative case, maintain connected the first device  which is not working and disconnect the rest of devices of the bus. Then, check thoroughly the end stretch of the wire.
  3. Common errors: crossroads of polarity, shorts-circuits and loose wires. 
  4. If is still not working, try a node that works isolated obtaining data directly to your PC. Repeat with the failing one. 



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