Just to Clear Things Up For Me

As i am pretty new to the electronics field I do not consider my self a PLC Master at all! I am pretty good at programming and troubleshooting them. I have no experience in networking. So that leads me to a few questions:

1. Embeded and PC based?
-from what i read, all i have experience with (S7 and AB) is all embeded. Would that be right

2. Modbus is an embeded controller?
-I think?

3. DCS(distributive control system) used vs. PID loop in PLC
-from what i understand it is faster responce time? why? and would this be a PC based system


Sorry for the "beginer" questions but i am confused.

Thanks to anyone who takes a stab at it.

Have you tried a Google on DCS and Modbus? I just did and there are tons of info out there. You could spend hours reading about it.

bigrthanur said:

Modbus is an embeded controller?

Click to expand...

Modbus is a serial communications protocol.

A protocol is a set of rules that defines
- the format of messages
- what kind of messages are acceptable or not
- who talks when and to whom,
- who listens when and to whom,
- how to whom responses are sent
- how to define errors and deal with errors.

In all communciations there is a physical layer and a protocol. The physical layer defines the elctrical characteristics and generally determines how far a signal goes, not the protocol. Typical physical layers for Modbus are RS-232, RS-485, ethernet, 232 over phone modems.

If we were to speak together on a phone line, the phone lines, handset,and switching office and interstate phone lines provide the physical layer. The dial tone, the busy signal, language we speak and manner in which we speak and respond are the protocol.

Theoretically, one could send Modbus by smoke signal as a physical layer, but it's not commercially viable.

bigrthanur said:

DCS(distributive control system) used vs. PID loop in PLC

Click to expand...

DCS's developed in and for the continuous process industries (refineries, paper, chemical) and have spread to the heavy industry batch processing (chemical), but are not used at all in discrete manufacturing. Hence, DCS's are heavy on analog I/O and hence PID, including various sophisticated PID algorithms.

bigrthanur said:

-from what i understand it is faster responce time? why?

Click to expand...

DCS's are not known for millisecond time response like a PLC can provide, although they do process a huge number of analogs multiple times (5x-10x)per second. It is not unusual nowadays to find PLCs integrated into sites that use DCS for those things PLCs do well.

bigrthanur said:

and would this be a PC based system

Click to expand...

There has been a shift towards Windows based HMI's on low & mid end DCS since the arrival of the NT/2K/XP kernel, but the controllers and I/O have been and will be dedicated, just like PLC systems.

Dan

Thanks Dan. That put a few ? to rest.

Thank you very much.

That was just the answers i was looking for!