Specifications

STB NIP 2311 Services
118
EIO0000000051 6/2010
What a Modbus Server Service Provides
A device that implements the Modbus server service (for example, the
STB NIP 2311 NIM) can respond to requests from any Modbus client. The Modbus
server service allows a device to make all its internal and I/O data available to
remote devices for both reading and control.
Modbus Function Codes
The Modbus protocol is a collection of function codes, where each code defines a
specific action for the server to perform. The ability of a device to perform read and
write functions is determined by the Modbus function codes that are implemented by
the server.
The Modbus protocol is based on five memory areas inside the device.
In addition to the function codes for reading and writing data within these areas,
there are codes for statistics, programming, device identification, and exception
responses.
When to Use the Client
A Modbus client should be used when data needs to be exchanged between two
devices at irregular or infrequent intervals such as when an event occurs. The client
allows a request to be triggered by the application code (in the case of a PLC or
SCADA) or by an internal timer (for a SCADA or an HMI). This allows you to initiate
communications only when required and provides a more efficient use of resources.
If the data must be exchanged at a short fixed rate, the I/O scanner service should
be used instead (if that service is supported by the client).
Memory Area Description
0x or %M memory bits or output bits
1x or %I input bits
3x or %IW input words
4x or %MW memory words or output words
6x extended memory area