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 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 (just 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.
