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
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.