Dwyer DPM - Manual (Page 42)

Particulate Monitoring Systems
Installation & Operating Manual
Document No. 210-1003-M Page 35 ©2007
Personal Computer Connection
Connection to a personal computer is made to the RS-232 COM port with the addition of a converter
module. Converter modules are available from the factory. The converter module modifies the voltage
levels and wiring connections to allow different RS connections to work together. Converter modules
generally require their own power source, which may be supplied with a wall-mount transformer
power supply.
Network Protocol Description – Modbus®/RTU
The communication protocol used to transmit data between network nodes is Modbus®/RTU
developed by Modicon. Devices communicate using a master-slave technique, in which only one
device (the master) can initiate transactions (queries). The other devices (slaves) respond by supplying
the requested data to the master, or by taking the action requested in the query.
When using our PC software packages the Modbus® protocol is coded into the software so that
configuration and operation of the network devices and software is simple. The user has no need to
know the specifics of the Modbus® protocol or the types of messages sent and received. All of the
low-level communications functions are taken care of and are transparent to the system user. The PC
running our software is configured as the network master and all other devices on the network are
configured as slaves.
Modbus® Message Description
The Modbus® protocol establishes the format for the master’s query by placing into it the device
address, a function code defining the requested action, any data to be sent, and an error-checking field.
The slave’s response message is also constructed using Modbus® protocol. It contains fields
confirming the action taken, any data to be returned, and an error-checking field. If an error occurred
in receipt of the message, or if the slave is unable to perform the requested action, the slave will
construct an error message and send it as its response.
Three data types are supported:
Discrete (1-bit)
Integers (16-bit)
IEEE Floating point (32-bit)
Modbus® Protocol Function Codes
Code Function Description
01 Read Coil Status Reads digital outputs or 1 bit data registers
02 Read Input Status Reads digital inputs or 1 bit data registers
03 Read Holding Register Reads analog outputs or 16 bit data registers
04 Read Input Register Reads analog inputs or 16 bit data registers
05 Force Single Coil Writes digital outputs or 1 bit data registers
06 Preset Single Register Writes analog output or 16 bit data register
07 Read Exception Status Reads status information
15 Force Multiple Coils Writes digital outputs or 1 bit data registers
16 Preset Multiple Registers Writes analog outputs or 16 bit data registers
17 Report Slave ID Reads device type information