Xylem YSI H-3553T, Owner'S Manual

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MODBUS OPERATION
28
Communication Setup
The Modbus communication interface is designed to
work with a Modbus Host device like PLC’s, etc. The
host must support RTU (Remote Terminal Unit) mode
to communicate with the H-3553T. Table 5-1 shows
the default settings required to communicate with
the H-3553T through the RS-485 Modbus interface
port. For more information regarding Modbus RTU
communication protocol and specifications see
www.modbus.org.
The H-3553T supports a Modbus client protocol interface. Modbus is an industry standard field bus for
interconnecting Programmable Logic Controllers (PLCs), intelligent sensors and other devices. The H-3553T
communicates Modbus via the RS-485 serial port connections; see Chapter 2 Table 2-1 for wiring connections.
This chapter will focus on Modbus setup and operation using the RS-232 menu interface or the SDI-12 interface.
Note : Modbus communication is only available when the system is not busy performing other requested
tasks (i.e. purging, automode with the rate set to zero, SDI-12 commands, etc.)
Setting Default Setting
Baud Rate 9600
Data Bits 8
Stop Bits 1
Parity Even
Protocol RTU
Table 5-1: H-3553T RS-485 Modbus Comm Settings
Function Codes
Modbus Function Codes are codes that are part of the Modbus command that specify what is being requested
via the Modbus protocol. The H-3555 supports two Modbus function codes. Table 5-2 shows the supported
function codes.
Function Code Description # of Registers
03 Read Holding Registers 41
16 Write Multiple Registers 41
Table 5-2: H-3553T Supported Modbus Function Codes
Holding Registers
The H-3553T has 41Holding Registers; these registers contain all the needed data for setup and measuring
the H-3553T. Table 5-3 is a list of the holding registers available.
Register Description Start Address # of 16-bit Registers Data Type
*ID String 0/ “0000” 16 Char String
Modbus Address 16 / “0010” 1 Short Integer
Stage Units 17 / “0011” 1 Short Integer
Reserved 18 / “0012” 1 Short Integer
Baudrate 19 / “0013” 1 Short Integer
Parity 20 / “0014” 1 Short Integer
Bubble Rate 21 / “0015” 1 Short Integer
Purge Pressure 22 / “0016” 1 Short Integer
Purge Sustain 23 / “0017” 1 Short Integer
Purge 24 / “0018” 1 Short Integer
Table 5-3: H-3553T Holding Registers