Explain Response time concerns of PLC

I am not able to clearly understand how the plc operates as explained in the Response time conerns page.
Thanks

What's 'the Response time conerns page'? where have you read it (guessing maybe one of the info pages on here?) and what PLC (is it a generic description?)

I am not certain what you are referring to regarding pages, however, here is an attempt to provide you with some information regarding your request.

You have the response time of your outputs, when initiated through the program. For a relay, it is typically 8 - 10 ms for the relay arm to go from a closed to open state or an open to closed state. Solid state outputs are typically faster than a relay.

A second possibility to what you are asking about is related to the scan time of a PLC, which includes the update of internal information. I can't speak for other manufacturers products, but with our hardware, the scan of a program goes first from left to right of the program rung, and then top to bottom of the program. (similar to reading english - left to right, top to bottom)

Some tend to word differently, when the update occurs. Basically, when the scan reaches the end of the program, it updates all internal information, registers, status of I/O, both external and internal. Once complete, it loops / returns to the top / beginning of the program and does the same thing all over again.

The above does not include immediate update I/O or high speed inputs tied to interupts.

Hi PeterW !
As u told I just started going through learn plcs option on plc.net.Here I found difficult to understand the scanning process or looping process of plc.

Hi Stephen Luft!
You almost cleared the picture of scan process of plc.I was asking just that,i.e how the internal process is done.
Thanks.

Stephen Luft said:

You have the response time of your outputs, when initiated through the program. For a relay, it is typically 8 - 10 ms for the relay arm to go from a closed to open state or an open to closed state. Solid state outputs are typically faster than a relay.

Click to expand...

To digress a little - a typical solid state relay has a switching time of about 1ms or less. Under high switching rates (say producing a vibration train) the solid state relays would be much more reliable. But in practice the operating time of your physical outputs is not a major concern to a programmer except for special purpose machinery (high speed filler etc).

Most common brands of PLC are built around a "scan" concept. Inputs are aquired at the start of a scan, and outputs are set at the end of the scan. The length of the scan is then the fastest. If you have a machine that needs to react within 100ms to a condition (say a prox being covered) when you have a scan time of 200ms, then your PLC has a response time problem.

Note that some newer platforms such as Allen-Bradley's ControlLogix are a little different. They can update their inputs at any time (not just at the start of the scan). However, they still set their outputs at the end of the scan, and hence the maximum rate of change of the outputs is the scan time.

In addition, some platforms also allow you to create an interrupt, which pauses the current scan while performing an action. These can be configured to come from a changing input, a clock, or an error in the PLC etc. They are also sometimes used when only a few points of IO require an immediate response without requiring a faster PLC overall.

Binaural said:

Note that some newer platforms such as Allen-Bradley's ControlLogix are a little different. They can update their inputs at any time (not just at the start of the scan).

Click to expand...

Wow, you saying that Bradleys have caught up with Siemens at long last!

geez, what a bunch of ****.

Respomse time, total, is the time it takes for the input to activate, the program to run, and the output to activate.

It is a mute point whether it is solid state or electromechanical...the response time is what it is.

The "scan" is just a part of the total response time whether an interrupt is used or not.

Stephen gave a good summation, what followed just confused the issue.

Hi Peter!
I am slowly getting what plcs are all about.Can u just give an idea of small project on plcs to concrete my little knowledge about plcs.