RLE LD1500 V.3.1 - Manual (Page 37)

rletech.com LD1500 User Guide 37
C
H A P T E R
C HAPTER 0
M
ODBUS
C
OMMUNICATION
This document describes the Modbus communications protocol as supported by the LD1500
Distance Read System. It includes details and information on how to configure the LD1500 for
communications via Modbus network.
5.1. Implementation Basics
The LD1500 is capable of communicating via the half-duplex EIA-485 serial communication
standard. The LD1500 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 LD1500 is a slave only
device and will never initiate a communications sequence.
5.1.1 Modes of Transmission
The Modbus protocol uses ASCII and RTU modes of transmission. The LD1500 supports only
the RTU mode of transmission, with 8 data bits, no parity and 1 stop bit. Every Modbus packet
consists of four fields:
♦ Slave Address Field
♦ Function Field
♦ Data Field
♦ Error Check Field (Checksum)
5.1.1.1 Slave Address Field
The slave address field is one byte in length and identifies the slave device involved in the
transaction. The valid address range is between 1 and 254. The slave address is set from the
Modbus/EIA-485 Configuration web page (see 4.3.7, “EIA-485/Modbus/N2” on page 32).