Installation manual

72
7 SERIAL COMMUNICATIONS AND
MODBUS RTU
7.1 GENERAL INFORMATION
The serial communications port on the S200 Position Node product uses an RS232
physical layer and communicates using a protocol known as Modbus RTU. The Modbus
protocol implementation of the S200 -CNS/ -DNS options is based on the on the
documents:
 MODBUS over Serial Line V1.0
 MODBUS Application Protocol Specification V1.1
Details of the protocol itself are not covered in this document. Both aforementioned
documents can be obtained at www.modbus.org.
This documents assumes that the user is knowledgeable about the information contained
in those documents and has some level of knowledge about serial communications, in
general. It is not the intent of this document to provide that basic information.
Section 7.6 will reiterate this but the Modbus strategy is kept simple: always 2-16 bit
word transfers (Defined by most interfaces as the ‘swapped mode’) using either integer or
floating point data types. The tables at the end of this section identify the data types for
each variable.
7.2 ABBREVIATIONS
Abbreviation Description
UA Unit Address
The unit address of the option card
configured through the rotary switches; S11
and S12.
FC ModBus Function Code (Defined by
standard)
CRC 16bit Cyclic Redundancy Check Value