Distech Controls ECLYPSE APEX, Руководство пользователя

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156 ECLYPSE APEX
CHAPTER 14
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 con-
troller’s datasheet for more information.
For controllers that support either BACnet MS/TP or Modbus RTU network options, this option is se-
lected in the controller’s web interface. For these controllers, BACnet MS/TP and Modbus RTU com-
munications are made by connecting directly to separate RS-485 ports. For example, the Connected
System Controller integrates up to three RS-485 ports when equipped with one ECY-RS485 extension
module allowing the controller to support more than one trunk or communication protocol at a time.
Modbus RTU Data Transmission Essentials
When the ECY Series Controller is configured for Modbus RTU, it acts as the Modbus master that initi-
ates 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-
gfx Program 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 indus-
trial 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 96 ).
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 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
ECB Series Controllers NET – NET + S
Table 27: Common Identification Labels for Modbus RTU Data Bus Polarity for Distech Controls’ Products
Modbus RTU Communication Data Bus Fundamentals