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PLC to DCS communicating.

PLC to DCS communicating.

PLC to DCS communicating.


Please excuse my ignorance here. My forty years experience lies with field instrumentation wired back to plc's or DCS systems.
I have no idea however, how a plc can talk with a dcs and vice versa.
Can anyone please help me with a nut-bolt explanation of what's used (hardware & software) or point me to a good web site or book ?

Any help is greatly appreciated.

Many thanks.

RE: PLC to DCS communicating.

Most all PLCs use a protocol called Modbus to communicate with everything. Modbus is essentially a flexibly defined tabular scheme. Communications become a simple communications handshake followed by exchanging various numbered values between tables. The tables can be bits, bytes, or larger integers.

With higher-end PLCs you can also communicate over a typical serial port using whatever coded comm scheme you come up with (proprietary). Lately most PLCs will also support communications over Ethernet.

Keith Cress
kcress - http://www.flaminsystems.com

RE: PLC to DCS communicating.

I mapped data into contiguous memory arrays in a ControlLogix PLC that a DeltaV DCS could read using Ethernet/IP. The DCS programmer just gave me the addresses to use. He did everything required to set up the comms and read the arrays.

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RE: PLC to DCS communicating.

Modern DCS's are OPC clients. An OPC client talks to an OPC Server. PLC vendors frequently have an OPC server as part of their HMI package or there are 3rd party OPC servers that have the driver that talks to the PLC in whatever protocol the PLC uses to communicate to the world. A classic OPC-DA server is a $800-$1000 software package (with the specific driver for your PLC) that runs on a Windows box. The OPC server is configured to get specific data items from the PLC. The OPC client finds any OPC servers on its network (which says that the Windows box has to be on the same network) and the server talks to the client.

If the PLC is a Modbus device, then the OPC server is configured with the Modbus addresses, engineering units and any conversion factor needed to interpret the Modbus data.

RE: PLC to DCS communicating.

Hi Weegie,
My first experiences, with DCS/PLC comms, date back to more than 30 years ago. In those days it was always Modbus over serial (usually RS-232 or RS-485). These days it is more likely to be Modbus over Ethernet (TCP/IP), but sometimes it is still serial.
Modbus is pretty simple to set up and is robust as well. The DCS is usually the Master and the PLC is usually the Slave. The Master will generally initiate all communications, and the Slave will respond to comm requests.
The Master will usually WRITE to a group of registers within the PLC. These 'write' registers are usually commands or setpoints. The PLC will in turn provide to the DCS READ command by providing access to a series of read registers. these 'read' registers are usually system status, measured variables, or some form of feed-back status.

When on doubt, check out Wikipedia;


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: PLC to DCS communicating.

Use Ethernet
Each PLC manufacturer uses a different application layer ( protocol on top of TCP/IP )
The DCS system should support many different PLCs.


Most all PLCs use a protocol called Modbus to communicate with everything
Actually only Modicon, Schneider and a few others like Automation Direct support Modbus. Modbus is a serial link. Don't use Modbus unless it is the only option because the protocol is very timing dependent.
Use Modbus TCP instead of Modbus RTU ( serial ).
The big systems usually use Siemens or Rockwell. Rockwell and Siemens PLCs don't normall support Modbus without buying special cards.

Peter Nachtwey
Delta Computer Systems

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