PLC vs. DCS

http://en.wikipedia.org/wiki/Programmable_logic_controller#PLC_compared_with_other_control_systems

The gist of this issue is found about midway through that Wiki: PID control loops.

PLCs can process a few, maybe a few dozen. But a processing plant such as a chemical manufacturing facility or a complex bio-pharma process may have hundreds or thousands of control loops. That is where a DCS will seriously out perform a PLC.

From the wiki article above

"As PLCs have become more powerful, the boundary between DCS and PLC applications has become less distinct".

A little bit of wishful thinking probably, remember that Wiki articles are written by regular folks and that one on PLCs was likely written by a PLC supplier. The Wiki article in DCS is likely written by a DCS supplier.

Still, from a hardware standpoint this is somewhat true, in fact Siemens' DCS system, the PCS7, is based on their S7-400 PLC hardware. But all similarities stop there. The computer interface capabilities and processing power of the DCS is far greater than what anyone would be willing to pay for in a PLC.

Dear all,

PLC for logic control such as limit switch input, motor start output, replacing hundreds of (or thousands of ) hard wire relays by only one PLD. For analog control, you may need an additional analog control board. It works much faster than DCS. So If your system is mainly logic control, use PLC.

DCS for analog control, such as temperature input and control valve position output, replacing hundred's of analog loop controllers. It can do the logic control same as PLC , but working slowly. If your system is mainly for analog loop control use DCS.

Regards

Thanks for the information, but the no of I/Os also works as deciding parameter for selection system such as PLC or DCS?

Dear all,

Number of I/O is not really related.

If the I/O is mainly on-off (binary) such as factory automation, PLC is selected.

If the I/O is mainly analog such as in process control in refinery, DCS is selected.

Regards

Thanks...