RLE Wi-MGR V.1.6 - Manual (Page 61)

rletech.com Wi-MGR User Guide 61
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H A P T E R
C HAPTER 0
M
ODBUS
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OMMUNICATIONS
4.1. Implementation Basics
The Wi-MGR is capable of communicating via the half-duplex EIA-485 serial communication
standard. The Wi-MGR is configured to act as a slave device on a common network. The
EIA-485 medium allows for multiple devices on a multi-drop network. The Wi-MGR is a
slave only device and will never initiate a communications sequence.
4.1.1 Modes of Transmission
The Modbus protocol uses ASCII and RTU modes of transmission. The Wi-MGR supports
only the RTU mode of transmission, with 8 data bits, no parity and one stop bit. Every
Modbus packet consists of four fields:
♦ Slave Address Field
♦ Function Field
♦ Data Field
♦ Error Check Field (Checksum)
4.1.1.1 Slave Address Field
The slave address field is one byte in length and identifies the slave device involved in the
transaction. A valid address range is between 1 and 254. The slave address is set from the
Com Port1/Modbus/Bacnet Configuration webpage.
4.1.1.2 Function Field
The function field is one byte in length and tells the Wi-MGR which function to perform. The
supported functions are 03 (Read 4xxxx output registers), 04 (Read 3xxxx input registers), 06
(Preset single register), 16 (Preset multiple registers).