Hello All,

I would like to know whether communication setup via OPC impacts PLC scan time in anyway? if yes could you please elaborate a bit, also I'll appreciate it much if some one can suggest an appropriate testing method to measure it myself in lab.

eg:-
A) scan time with out OPC - PLC running and a few input and output modules connected(communicating)

B)scan time when I configure OPC server to read all input/outputs at the fastest possible update rates.

Any helps/hints would be greately appreciated.

thank you
rajeesh

 

PLCs don't use OPC to communicate with a PC. They use their own protocol such as Modbus, Profibus, Devicenet, etc. However, many HMI and SCADA programs use OPC to communicate to their own communications driver. In other words, OPC is not an industrial communications protocol. It is simply an interface between certain PC programs and communications drivers that they use. It's a way of linking two programs together using Microsoft's COM/DCOM remote procedure call methods for MS Windows.

The PLC has no idea that OPC even exists. If the HMI or SCADA program understood the PLC's protocol, it wouldn't need OPC at all.

Communications effects on PLC scan time should be documented in the manuals for your particular PLC. Whether there is any scan delay will depend on the design of the PLC, the protocol which the PLC uses (which *will not* be OPC), whether the communications is handled by the same CPU as is running the logic program or has a dedicated processor, the amount of data being transferred, and no doubt many other factors.

 

This exactly what I wanted to know. I understand that PLC really have no idea about existence of OPC server. Could you please give me names of some PLCs having dedicated processor for external communication?

Thanks a lot for your reply Griffin.
Best Regards,
Rajeesh

 

Many models of PLC accept a communications module or card which plugs into the rack or into a expansion connector. There are often different versions of module which support different protocols. If you look in the advertising literature for the PLC brand that you are interested in, you should find a selection of modules.

Some PLCs also have an internal communications card that has its own microprocessor. That is usually an undocumented design feature and not something they advertise. It is also something that can be eliminated from their design as a cost saving if they upgrade the main microprocessor to a faster model. If you want to find out which PLCs do this, you will have to do some digging for yourself as I haven't kept any notes as to which ones are designed like this.