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9.0 SERIAL COMMUNICATIONS AND
MODBUS RTU
9.1 G
ENERAL 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 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.
9.1.1 Abbreviations
Abbreviation
UA
FC
CRC
9.2 RS232 S
The S200 Position Node defaults the serial port settings as follows:
Baud rate:
Data bit:
Stop bit:
Parity:
8.3 E
XCEPTION
The Modbus protocol defines exceptions (error identification methods). The S200 implements
exceptions according to the requirements and defines additional exception codes. The following
table lists all proprietary codes.
53
P
C
ERIAL
ORT
38400
8
1
Odd (can be changed to Even using S13 Switch 3)
M
ESSAGES
Description
Unit Address
The unit address of the option card
configured through the rotary switches; S11
and S12.
ModBus Function Code (Defined by
standard)
16bit Cyclic Redundancy Check Value
ONFIGURATION
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