RS485 network requirements

Use only 3-wire RS485.

  • Please wire your BMS to the RS485 port (DB+ DA- GND) of our interfaces for Modbus communication.
    • Do not wire a different communication standard to the RS485 port. 
    • To verifiy the RS485 transceiver of our interface is operating correctly:
      • When our interface is disconnected from RS485 network (only power is connected), you should see about 2V between pins DA and GND and about 2V between pins DB and GND. 
  • If a communication standard other than RS485 is used then you will need a converter to the RS485 standard (*for RS232 standard see info about RTD-DSX below).
    • Example: For connection to a PC use a USB-to RS485 adapter (when testing with our software ModTool we recommend the FTDI USB-RS485-WE-1800-BT available from RS Components, RS Stock No. 687-7834)

 

The RS485 network requirements are:

  • RS485 Cables must use stranded 24awg shielded or unshielded twisted pair to Cat3, Cat4 or Cat5 (and Cat5e) specification.
  • Use a twisted pair for connections DB,DA and an extra core for connection GND.
    • The twisted pair cable must connect terminals DB(+) and DA(-) on each RTD.
      • Our interfaces follow the RS485 standard. A and B ports are the + (positive) and – (negative) ports, respectively.  *It is important that you check in your RS485 module's manual which of the A and B ports is the + (positive) and – (negative port, respectively, as not all devices follow the RS485 standard in this respect.
    • Terminal DB must be connected to all other DB terminals.
    • Terminal DA must be connected to all other DA terminals.
    • In addition the common terminal GND on all devices must be connected together. If a shielded cable is used then the shield can be used for this purpose. It is recommended that the GND connection is connected to local Earth at one point only.
    • The network must be installed as a daisy-chained point-to-point Bus configuration, Star and Ring connections must NOT be used (see the drawings in our interfaces' installation instructions for clarification).
  • RS485 network length < 500 m. 
    • Standard installation for total network distances of up to 500m can be achieved following the basic daisy-chaining method. The network can be extended further using RS485 repeaters.
  • Generally, a resistance termination is not required. 

 

RTD-DSX: We also offer a Modbus gateway which has a RS232 port (if preferred over RS485). Please follow this link for further information.