Appendix C-Modbus Plus Communication Between An Abb Protective Relay And A Modicon Plc - ABB TPU2000 Technical Manual

Appendix C - Modbus Plus Communication Between an ABB Protective
ABSTRACT:
Modbus Plus Capable devices are continuously being introduced into the utility environment. This
Application Note is intended to educate the user with the method to use the protective relay and PLC's Modbus
Plus capabilities to allow for data access and control capabilities. A simple communication example is intended to
give the reader a simple method to establish communication between devices using PLC Ladder Logic. This
Application Note relies upon the reader's understanding of Ladder Logic programming of a Modicon PLC and
Modbus Plus application.
Modbus Plus General Information
Modbus Plus is a communication protocol, which encompasses the physical layer, data link, transport and
application link layer definition within the ISO model representation. The physical layer is a hybrid-defined
interface, which allows up to 64 devices to be multi-dropped along a serial interconnection. The interface also
allows devices to communicate to each other with a data rate (baud rate) of 1 megabaud. The combination of
protocol implementation and baud rate selection make Modbus Plus an excellent high performance protocol
desirable for the substation environment.
However, Modbus Plus has been given additional capabilities, which exceed the benefits of a fast baud rate.
Modbus Plus is based upon a hybrid implementation of HDLC (High-level Data Link Control) protocol. This
implementation allows multiple devices to communicate along a single cable interface. Modbus Plus allows up to
32 (or 64 with the addition of repeaters) devices to communicate along a network connection. Additionally, each
device can be capable of transmitting/receiving data of a length of 32 data words, which can be seen by all nodes,
attached to the network. Modbus Plus has the Modbus Protocol imbedded within its data transport structure.
Modbus Plus is a deterministic network in that the response time to a command can be reasonably calculated.
The method of determinism employed is referred to as "Token Passing". Please refer to both ABB and Schneider
Electric documentation referencing network throughput calculation. Each node attached to the network can
read/write information in a calculated amount of time which is determined by data transferred along the network
and the amount of nodes along the network receiving the token. The amount of time in which to transfer the
token to each network is referred to as "Token Rotation Time".
Modbus Plus Communication Between Devices
Modicon Programmable Logic Controllers (PLC's) can communicate with Modbus Plus Capable devices using
two methods: Method 1 is using Peer Cop,. Method 2 is using a Master Block.
Peer Cop is a capability used only to allow devices by Schneider Electric to communicate with each other. It
enables one device to be configured to read or write from/to each other along a Modbus Plus connection path.
It's throughput is dependent upon the Modbus Plus token rotation time.
A Master Block is the PLC's method of using an instruction, which is inserted within the PLC's Ladder Logic scan
to access data from another Modbus Plus capable device. A Master Block performs the following data access
tasks:
• Write 4X Data to other devices
•
Read 4X Data from other devices
• Get Local Modbus Plus Network Statistics
•
Clear Local Modbus Plus Network Statistics
• Write Global Database
•
Read Global Database
• Get Remote Modbus Plus Network Statistics
•
Clear Remote Modbus Plus Network Statistics
•
Obtain Node Peer Cop Health Statistics
TPU2000/2000R Modbus/Modbus Plus Automation Guide
Relay and a Modicon PLC
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