Question for SCADA programmers

I have a few questions about how SCADA and a PLC share data. For example, when a SCADA sends a setpoint value to the PLC, does it send only once or does it continue to brodcast the setpoint. As an example, say an integer setpoint is sent via SCADA to a PID loop. Let's say the value is 400 gpm. On the OIT which is connected to the PLC, the same setpoint can be entered. If a value of 300 gpm is sent via the OIT, it just sends the data to the PLC register once. Will the SCADA overwrite it to 400 or will it remain at 300?
How about bits? If a pushbutton on the OIT is set up as maintained, can the SCADA change the state of the PLC bit which the OIT addresses?
These questions are geared toward Wonderware and Intellution systems and SLC 500 PLCs but not to any particular project.

In general, when SCADAs or OITs write numeric data to a PLC, they only write it once when you hit 'ENTER' or otherwise indicate that you're satisfied with the number. Since nimeric memory in the PLC is retentive, the number will remain unchanged unless something else changes it. The something else could be another SCADA or OIT, or it could be the ladder logic program.

The same generally holds for bits that the OIT or SCADA might write to the PLC. If an object on the SCADA or OIT screen is set up to be 'momentary', that usually menas that the SCADA writes a value of 1 to the bit when you touch the object (or on the 'mouse down' event). It then does nothing further until you take your finger off the object ('mouse up' event) at which point it writes a value of 0 to the bit.

If you set up the object as 'maintained', the OIT omits writing the value of 0 to the bit when the operator releases the object.

If you set up the object as 'toggle', the OIT first reads the current state of the bit and writes back the opposite state.

Note that these are generalizations. Specific details may vary among SCADA packages or OIT models.

Its thew same has if you where entering data directly in a PLC through another device. It does it only once.

BUT...

Therer are other way to do it in order to satisfy your needs. Just like in any PLC logic.

IF you want to have a Setpoint rewritten in the PLC, so it will not be change by another device, you must have some logic within the SCADA.

Example:

You want Setpoint "A" to remain what it is set by SCADA X.

Write the Setpoint value in an internal "memory register" of the SCADA.

ADD the value of ZERO to it.

The result goes to the PLC register.

This value will be written according to your choice. Always (slows down the r/w time) is the choice for this setup.

Same ting goes for the bit, latched or not.

A lot depends on the SCADA system and the programmer. I found one job where the programmer meant to write to the PLC to turn on a bit. He used the sleep command in his code but had it set far too long. The PLC was unable to turn it off and the sleep command caused an unholy slow down in the SCADA system. Basically the whole thing went to sleep.
Code can be written in SCADA systems to do whatever you wish either in a script language or VB. Some now use Java. I prefer to use the inbuilt script as it generally works better.
Also, prefer Citect. Particularly for trending. Their trending is the best I have seen up till now but have not used the latest versions of many SCADA systems so they may have been improved a lot.
beerchug