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2103 Modem Module
Section 5 Modbus Protocol
5-4
5. OPC connects to the 2150 stack through the cable (direct
connection), takes register data from the specified 2150,
and populates the OPC server's holding index.
6. Process Control takes data from the OPC server's holding
index and gives data to the user.
Note that Process Control can be either manual or automated in
this example, and that the OPC server and Process Control may
be located physically on the same computer.
5.4 Glossary of Terms ASCII – Short for American Standard Code for Information
Interchange, ASCII is a code that represents English characters
with numbers. Most computers represent text with ASCII code,
making it possible for one computer or device to share data with
another.
2100 modules support Modbus ASCII protocol.
Dedicated Line – A telecommunications path reserved for com-
munication between two specified points and not shared among
multiple points.
Modbus Protocol – Modbus Protocol is a messaging structure
used to establish master-slave/client server communications
between intelligent devices. Modbus is a simple
command/response mechanism to read from and write to reg-
isters.
OPC – OPC (OLE for Process Control) means open connectivity
via open (free for use) standards. It is a series of software stan-
dards specifications that fill a need in automation (like printer
drivers did for Windows), acting as a translator for data trans-
mission and process control.
The specification defines a standard set of objects, interfaces, and
methods for use in process control and manufacturing auto-
mation applications to facilitate interoperability. There are hun-
dreds of OPC Data Access servers and clients.
Registers – Registers are locations in memory that have specific
data stored for retrieval or are used for control functions. A reg-
ister is a holding place for a piece of digital information within
the equipment. The definition of what is contained and where
(the registry number, or address) is decided by the manufacturer
(in this case Teledyne Isco).
SCADA – SCADA (Supervisory Control And Data Acquisition)
is a computer system for gathering and analyzing real-time data.
SCADA systems are used to monitor and control plant operation,
or equipment in industries such as telecommunications, water
and waste control, energy, oil and gas refining, and transpor-
tation.
The SCADA system transfers the information (for example,
where a leak has occurred in a pipeline), back to a central site,
alerting the home station of the leak, performing necessary
analysis and control (such as determining if the leak is critical),
and displaying the information in a logical and organized
manner.