Steps for Modbus implementation in Siemens S7-300 PLC System are shown where CPU is configured as Modbus Server.

1. Modbus Communication for
Slow Controller
INTER‐CONTROLLER COMMUNICATION

2. Agenda
Introduction
Standard Protocols
What is Modbus?
Modbus in Detail
Comparison Modbus and OPC
Implementation of Modbus for Siemens PLC
Security in Modbus
Conclusion

3. Introduction
Slow controller (PLC) and Fast Controller(PXIe) system are the two key controllers of Local control unit for the
Gyrotron system.
All auxiliary systems and power supplies are to be controlled by PLCs. Sequence control and soft interlock
functions are to be implemented in PLCs.
High voltage and Magnet power supplies, Fast Local Protection and Data Archiving are performed by Fast
Controller.
Hence, Many process variables will be exchanged between slow and fast controller for the operation of Gyrotron.
So Standard communication protocols will be used for the same.
In this presentation, various protocols available are mentioned and Compared. Finally Modbus communication is
discussed mainly.

4. Available Protocols
 OLE for Process Control (OPC)
 Modbus Communication
 Direct Access
Standard
Non‐Standard
Industrial Communication
(Modbus)
Direct Access
OPC

5. What is Modbus?
It is simple, reliable and efficient Communication protocol across a variety of controllers.
Application layer messaging protocol for client/server communication between devices
connected on different types of buses or networks
Server/Client protocol and offers services specified by function codes.
(Client) (Server)
MODBUSMODBUS
RS485RS485
RS232RS232
TCP/IPTCP/IP

6. MODBUS TCP
•MODBUS TCP is Client/Server serial communication that uses TCP/IP as a transmission medium
•The client establishes a connection and sends request frames to the server. The server processes
this request and sends the appropriate answer to the request back to the client.
•The MODBUS protocol defines a simple Protocol Data Unit (PDU) independent of the underlying
communication layers. The mapping of MODBUS protocol on specific buses or network can
introduce some additional fields on the Application Data Unit (ADU).

7. MODBUS Data Types

8. Modbus TCP Implementation in STEP‐7
For Siemens PLC, Modbus can be implemented in
◦ Integrated CPU PN Port
◦ Extra Communication Processor PN Port(CP‐343‐1 Lean)
Siemens provides additional function blocks for Modbus TCP via PN CPU. These blocks are to be
purchased and licensed for one particular CPU only.
◦ FB102, FB103,FB104,FB105, DB2
These blocks are then to be configured as per the user application.
◦ Server or Client,
◦ Local IP and Port settings
◦ Memory area allotment in CPU
◦ Data Types of Variables to be exchanged
Siemens PLC, as an Modbus server, defines a stack of memory area where all incoming and outgoing
data will be stored.

9. Program Structure

10. Modbus Parameter Settings

11. Test of Communication : PLC Server
oData which is to be
transferred is stored in
DBs.
Similar, Data from
Other controller is
stored here in the
same DBs at particular
address
PLC is set as ModBUS
server at IP address
192.168.23.239 and
Port No. 502
Variable to be
Communicated
MODBUS Connection
Settings

12. Test of Communication : PC Client
oThis is computer
based Modbus
client. But it could
be any other
controller capable
of Modbus
communication.

13. Comparison
Parameters OPC Server Modbus TCP
Open standard Open Protocol
Extra Software 1. Vendor dependent servers are
available
1. No special software needed
Use of PC 2. Use of PC is needed to install it.
PLC talks to OPC server PC and
then PC talks to FC
1. No PC is Required. PLC directly
talks to FC
Security 3. Less as OPC server can access
any memory area of PLC e.g.
input, output, internal registers
w/o any information to PLC CPU.
1. Only those data which PLC
intentionally wants to transfer are
accessed by remote controllers.
PLC knows what it is sharing with
other controller

14. Continued…
Parameters OPC Server Modbus TCP
Operating Systems Windows based standard ‐
Amount of work Quite less More, so suitable for one
time installation
Flexibility and upgradability Quite Simple Much effort needed.
Application dependency? Independent of process
Application
Dependent of process
application.
Debugging? What data available, in which
form, at what place available
More effort needed.
Cost High, depends on no. of tags, Quite less, no variable
limitation

15. Summary
MODBUS is open protocol which can be used across variety of networks and between variety of
controllers and/or PC
Even though it is inferior to OPC standard communication, but for small amount of process
variables exchange directly between two controllers, this protocols can be adopted which
eliminates need of OPC computer application.
Time and speed response of Modbus is comparable to that of OPC communication (~100ms)
Additional Modbus Blocks from Siemens need to be purchased and can be used for one
particular CPU only.