PLC to DCS communicating.
PLC to DCS communicating.
(OP)
Hi.
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.
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.
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.
xnuke
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RE: PLC to DCS communicating.
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.
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;
https://en.wikipedia.org/wiki/Modbus
Regards,
GG
"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.
Each PLC manufacturer uses a different application layer ( protocol on top of TCP/IP )
The DCS system should support many different PLCs.
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
http://www.deltamotion.com
http://forum.deltamotion.com/