Modbus RTU Communication Data Bus Fundamentals
ECLYPSE User Guide 155
Modbus RTU Data Transmission Essentials
The ECY Series Controller can support either BACnet MS/TP or Modbus RTU network on its
RS-485 port. This option is selected in the controller’s web interface. When the ECY Series
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 ECY Series Controller does not work with Modbus ASCII
devices.
The Modbus network communication parameters and the Modbus device parameters are
configured with EC-gfxProgram in the Resources Configuration window, Modbus Device
block.
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-available 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 (see Figure 14-5).
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 usually specified by the cable
manufacturer.
Modbus RTU Data Bus is Polarity 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.
Table 14-1: Common Identification Labels for Modbus RTU Data Bus Polarity for
Distech Controls’ Products
Typical Data Bus Connection Terminals
Table 14-2: Common Identification Labels for Modbus RTU Data Bus Polarity for
other Manufacturers
Typical Data Bus Connection Terminals
Common identification labels for Modbus
RTU data bus polarity by other
Manufacturers
When interfacing with Modbus RTU devices from other manufacturers, refer to the
documentation provided with the device to correctly wire the device.
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