CHAPTER 13
Modbus RTU Communication Data Bus
Fundamentals
This chapter describes the Modbus RTU Communications Data Bus operating principles.
Controller Modbus Support
Certain ECLYPSE controller models support communication with Modbus devices. Refer to the controller’s datasheet for
more information.
For controllers that support either BACnet MS/TP or Modbus RTU network options, this option is selected in the con-
troller’s web interface. Modbus RTU communications are made by connecting directly to the RS-485 port.
Modbus RTU Data Transmission Essentials
When the ECLYPSE Controller is configured for Modbus RTU, it acts as the Modbus master that initiates requests to any
slave device connected to this data bus. All slave devices must support Modbus RTU communications protocol. The
ECLYPSE Controller does not work with Modbus ASCII devices.
The Modbus RTU data bus protocol uses the EIA-485 (RS-485) 3-wire physical layer standard for data transmission.
EIA-485 is a standard that defines the electrical characteristics of the ECLYPSE Wi-Fi Adapters and drivers to be used to
transmit data in a differential (balanced) multipoint data bus that provides high noise immunity with relatively long cable
lengths which makes it ideal for use in industrial environments. The transmission medium is inexpensive and readily-avail-
able twisted pair shielded cable.
While there are many possible LAN topologies for an EIA-485 data bus, only devices that are daisy-chained together are
allowed with Modbus RTU.
End-of-line (EOL) terminations are critical to error-free EIA-485 data bus operation. The impedance of the cable used for
the data bus should be equal to the value of the EOL termination resistors (typically 120 ohms). Cable impedance is usu-
ally specified by the cable manufacturer.
Modbus RTU Data Bus is Polatiry Sensitive
The polarity of all devices that are connected to the Modbus RTU data bus must be respected. The markings to identify the
polarity can vary by manufacturer. The following table summarizes the most common identification labels for Modbus RTU
data bus polarity.
Controller Typical Data Bus Connection Terminals
Inverting Non-inverting Reference
Common identification labels for
Modbus RTU data bus polarity
D0 D1 SC, C, or C
A or A’ B or B’ Common
Data – Data + Data 0V
Table12: Common Identification Labels for Modbus RTU Data Bus Polarity
When interfacing with Modbus RTU devices, refer to the documentation provided with the device to correctly wire the device.
Modbus RTU Communication Data Bus Fundamentals
110
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