M
Michael Griffin
In reply to Dick Caro: You have qualified your points a bit, but I think that it needs to be pointed out that "no time-critical I/O points and remote I/O is not being used for a safety application" covers most of the market.
I would also like to point out that almost all of the I/O available for the networks you mentioned ("Modbus/RTU, PROFIbus, Interbus, ASi, DeviceNet,
ControlNet, or EtherNet/IP") *CANNOT* be used for safety applications either.
There is more to a safety system than just determinism. There are safety versions of Profibus and ASI (and perhaps others as well), but the entire system from end to end (PLC program, PLC, network, and I/O) must all be made
with special safety versions, not the garden variety components.
As for a "need for the PLC to react in 'real-time'", most PLCs themselves are not "real-time". Furthermore, I think a more typical application for this type of I/O is with PC based automation, and very few PC applications require real-time either. When the I/O is used over a closed network, transmission delays will be negligable compared to the delays in the PLC/PC or in the I/O devices themselves.
While I agree with your points in principle, I think you have placed undue emphasis on certain marginal applications (networked safety and real-time) and not enough on normal machine control and monitoring.
I would also like to point out that almost all of the I/O available for the networks you mentioned ("Modbus/RTU, PROFIbus, Interbus, ASi, DeviceNet,
ControlNet, or EtherNet/IP") *CANNOT* be used for safety applications either.
There is more to a safety system than just determinism. There are safety versions of Profibus and ASI (and perhaps others as well), but the entire system from end to end (PLC program, PLC, network, and I/O) must all be made
with special safety versions, not the garden variety components.
As for a "need for the PLC to react in 'real-time'", most PLCs themselves are not "real-time". Furthermore, I think a more typical application for this type of I/O is with PC based automation, and very few PC applications require real-time either. When the I/O is used over a closed network, transmission delays will be negligable compared to the delays in the PLC/PC or in the I/O devices themselves.
While I agree with your points in principle, I think you have placed undue emphasis on certain marginal applications (networked safety and real-time) and not enough on normal machine control and monitoring.