How DCS Scans?

Hi Folks,

As with PLC scan cycle it looks for inputs,executes the algorithm and write the outputs..Is this same for DCS? or please let me know if DCS has some other way to do the scanning.....

Best Regards.
Duncan...

It's not even what you described for some PLC's. Many PLC's are now asynchronous, and handle the I/O update in a sequential manner, without any lock-stepping to program scan.

You need to check the system documentatin to be sure.

A "classic" DCS consists of essentially analog controllers that "continuously" execute their control algorithm. This is typically PID control. Hardware in the individual controllers also does alarm limit checking on an essentially continuous basis. The DCS simply communicates with the individual controllers periodically reading values and status and wiritng setpoints and tuning.

New controllers are digital, of course, but still emulate the continuous functioning control algorithm. The reading of data, algorithm solving, and writing outputs is going to be separate steps but will be essentially transparent. Reading values over the comm link will still be periodic.

DCS Scanning?

I am quite confused between DCS and PLC scanning procedure..
Please help me in understanding the concept.....
In DCS we can set different scan times for modules(PID block,motor control block).Now In relation to this how the DCS scanning proceeds?
Is it like first scan inputs,then look for the modules and scan them as per their scan time given,execute algorithm and finally write outputs?

Duncan

I'm only familiar with the Foxboro I/A DCS system. In Foxboro each I/O module is setup using an ECB (Equipment Control Block), this block sets the scan rate for the I/O. The control blocks ( PID, Device control, math etc...) read the I/O when scheduled, so I/O updates are independant of control block scan rates.

So...if you had an I/O module with a 1 second scan rate, when the control block reads it, it could be anywhere from 0 to 1 seconds old.