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FS-8700-01_Modbus_RTU_(T28700-01) Driver Manual Page 3 of 21
FieldServer Technologies 1991 Tarob Court Milpitas, California 95035 USA Web:www.FieldServer.com
Tel: (408) 262-2299 Fax: (408) 262-2269 Toll_Free: 888-509-1970 email: support@FieldServer.com
1. Modbus RTU/ Modbus ASCII Description
The Modbus RTU and Modbus ASCII drivers allow the FieldServer to transfer data to and from
devices over either RS-232 or RS-485 using Modbus RTU or Modbus ASCII protocol
respectively. The driver was developed for Modbus Application Protocol Specification V1.1a"
from Modbus-IDA. The specification can be found at www.modbus.org. The FieldServer can
emulate either a Server or Client.
The information that follows describes how to expand upon the factory defaults provided in the
configuration files included with the FieldServer.
There are various register mapping models being followed by various vendors
To cover all these models FieldServer uses the following three Models
• Modicon_5digit – Use this format where addresses are defined in 0xxxx, 1xxxx, 3xxxx or
4xxxx format. A maximum of 9999 registers can be mapped of each type. This is
FieldServer driver’s default format.
• ADU –Application Data Unit address. Use this format where addresses of each type are
defined in the range 1-65536
• PDU –Protocol Data unit address. Use this format where addresses of each type are
defined in the range 0-65535.
The key difference between ADU and PDU is for example if Address_Type is ADU and address
is 1, the driver will poll for register 0. If Address_Type is PDU, the driver will poll for address 1.
Note 1: If vendor document shows addresses in extended Modicon (i.e. 6 digit) format like
4xxxxx then consider these addresses as xxxxx (omit the first digit) and use either ADU or PDU
Note 2: The purpose of providing 3 different ways of addressing the Modbus registers is to allow
the user to choose the addressing system most compatible with the address list being used. At
the protocol level, the same protocol specification is used for all three with the exception of the
limited address range for Modicon_5digit.