User guide

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A PPENDIX
CHAPTER 0MODBUS COMMUNICATIONS
A.1. Implementation Basics
The Wireless Gateway is capable of communicating via the half-duplex EIA-485 serial
communication standard. The Wireless Gateway 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 Wireless Gateway is a slave only device and will never initiate a communications
sequence.
A.1.1 Modes of Transmission
The Modbus protocol uses ASCII and RTU modes of transmission. The Wireless Gateway
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)
A.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
Modbus/EIA-485 Configuration webpage.
A.1.1.2 Function Field
The function field is one byte in length and tells the Wireless Gateway which function to
perform. The supported functions are 03 (Read 4xxxx output registers).