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We are having 10 third party PLC systems to be connected to DCS. We wanted to use ModBus TCP/IP protocol. For this, OPC server facility is required or the PLCs will act as ModBus TCP/IP Server. Please clarify.

If OPC server shall be used, the cost would go high?

If your DCS and your PLCs are all capable of communicating in Modbus TCP/IP you shouldn't need a server. The DCS should be able to use the PLC as though it were remote I/O.

Roy

If your DCS talks Modbus TCP as a Modbus master directly (a likely scenario) the DCS can talk directly to PLC slave devices assuming the PLCs can be Modbus TCP slaves.

For OPC, let's assume your DCS is an OPC client (also a likely scenario). An OPC server, with the appropriate "drivers", is used to communicate to your PLCs.

In OPC, an OPC server generally resides (not always, but generally) on a network server box, not on the PLC. The OPC server communicates with the field devices, the PLCs, over whatever physical layer is available, RS-485, Ethernet, whatever using the specific 'driver's' protocol to talk to the PLC. It is not uncommon for an OPC server to talk to a PLC using Modbus TCP or Modbus RTU. The OPC server's drivers can be other than Modbus though.

The OPC server then reports its data (on an exception basis) to the OPC client, typically over Ethernet through a DCOM tunnel or directly via COM.

If your PLCs have Modbus TCP server cards you don't need a OPC server. If your PLCs don't "speak" modbus then you need an OPC server that acts as an translator between your DCS and your PLCs. If your PLCs and the DCS are from the same manufacturer the integration is even easier provided that DCS and PLCs "speak" the same protocol.

As you see, the solution depends on the compatibilities between DCS and PLCs.

I'm assuming that your 3rd party PLC currently communicate through Serial MODBUS. In that case you need a protocol converter (MODBUS RTU->MODBUS TCP/IP) which will convert it to MODBUS TCP/IP. If the DCS is capable of communicating directly to MODBUS TCP/IP, then you don't require anything else. If not, then you need another protocol converter or an OPC server (with MODBUS TCP/IP driver) to interface with the DCS.

If my assumption above is not correct(about your PLC communication capability), then try to visit MOXA website. They have a bunch of protocol converters that you can use for your project.

OPC won't drive your cost up at all (note the price in the link below). The OPC server for Modbus is really cheap. Also it can be configured for the 10 PLCs in just three steps. You can download it and try it for 30 days:

http://www.matrikonopc.com/opc-drivers/128/base-driver-details .aspx

Let me know how it goes for you

Wassim Daoud
Global Solutions Architect
MatrikonOPC

Please mention the PLC brand and DCS brand so that we will be able to guide you