User manual

Ethernet TCP/IP Modbus Application Protocol
344
33002479 06 07/2008
Modbus Application Protocol Overview
Introduction The following information describes the Modbus Application Protocol (MBAP).
The Modbus Application Protocol (MBAP) is a layer-7 protocol providing peer-to-
peer communication between programmable logic controllers (PLCs) and other
host-based nodes on a LAN. Collectively, these nodes implement all or part of a
control application used for industrial automation applications in the automotive, tire
and rubber, food and beverage, and utilities industries, to name a few
Client-Server
Communications
Modbus protocol transactions are typical request-response message pairs between
a client node and a server node. These nodes function as follows
Client : The node that initiates a data transaction is called a client. The Modicon
Quantum Ethernet module provides the user with the capability to transfer data
to and from nodes on a TCP/IP network using a communication instruction. All
PLCs that support networking communication capabilities over Ethernet can use
either the MSTR Ladder Logic instruction to read or write controller information or
IEC communication blocks.
Server : The node that receives an inquiry is the server. Using the standard
Modbus/TCP protocol, the Modicon Quantum Ethernet module provides access
to controller data. Any device, PC, HMI package, another PLC, or any Modbus/
TCP compliant device can access data from the PLC. The Modbus/TCP server
also allows programming panels to log into the controller over Ethernet.
Modbus requests contain function codes representing several classes of service
including data access, online programming, and program download and upload
classes. Modbus responses can be ACKs with and without data, or NACKs with
error information.
The Modbus Application Protocol can be transmitted over any communication
system that supports messaging services. However, the current Quantum
implementation transports Modbus Application Protocol PDUs over TCP/IP. The
Quantum PLC accommodates both Ethernet II and IEEE 802.3 framing, although
Ethernet II framing is the default.