PLC CPU Speed?

Dhornberg · Apr 24, 2012

This might be a stupid question but here we go anyway:
If you look at product details of PLCs there are never any information about CPU speed. I'm writing a report and I'd really like to give some examples of different PLCs' CPU speed (in Hz).

Is there a reason they never mention CPU speed? In some cases (Mitsubishi for example) they mention that "Program cycle period 9.5 ns/log. instruction". Can i use this to calculate CPU speed?

JesperMP · Apr 24, 2012

To my experience, the manufacturers brochure information is near useless to gauge CPU speed.
Better is to compare identical instructions using the PLCs instruction references. You have to compare a mix of boolean, integer, floating point and program control instructions. Even so, it will not be 100% correct.

Dhornberg · Apr 24, 2012

Ok. But every PLC must have a CPU clock with a certain clock rate right? I mean even if it doesn't tell much about the actual program execution speed there must be a fixed number for the CPU clock rate?

The reason i'm asking is because i want to write something along the line of "An average PLC has a CPU-clock frequency of about 100MHz while a modern PC often has a CPU-frequency of 3GHz!"
I made up the numbers now, but you get my point.

JesperMP · Apr 24, 2012

The manufactureres NEVER tell you the details about the CPU, such as what microprocessor, what clock speed, what backplane bus speed etc.

Your best bet for an objective comparison is to go by the instruction timing from the manufacturers reference literature. And backed up with some real measurements.

Dhornberg · Apr 24, 2012

Allrightie

Well thanks for replying!

rdrast · Apr 24, 2012

It's a useless data point anyway, as demonstrated over the past decade of the PC 'Clock Wars'. Once upon a time, with single core, single threaded processors, the clock speed had some meaning. Now, with multiple execution pipelines, multi-threading, multi-core CPU's, and decoupling memory and I/O from the cores, pure clock speed means almost nothing.

JesperMP · Apr 24, 2012

Also, in the not so distant past every micro-second would count, since the CPUs were not so powerful. Today, the CPUs are 100x as powerful and there is "enough" CPU power. In some cases, you still have to consider the CPU performance, but it is not such a critical issue as it used to be.

Doug_Adam · Apr 24, 2012

Schneider Electric (Modicon) publishes the clock speed of their Quantum range PLCs.
Also GE published the clock speed of their new PACs based on intel Atom, IFM along with all the other mobile controller type PLCs usually publish the clock speed of their PLCs.
Many don't though.

Peter Nachtwey · Apr 24, 2012

There are all sorts of things that affect speed. Memory width and cache size haven't been mentioned. Few people ask us how fast our motion controller processor goes.

Also, CPU speed and higher clock speeds in general usually mean more heat.

The efficiency of the code in the firmware can make a much bigger difference than a few clock cycles.

Austin1970 · May 24, 2012

Anybody ever tried to alter the clock speed of a RS Logix 5000? Would it be possible to do it thru programming?

alan_505 · May 25, 2012

Why would you want to?????

rdrast · May 25, 2012

Austin1970 said:
Anybody ever tried to alter the clock speed of a RS Logix 5000? Would it be possible to do it thru programming?

You can alter the clock speed of RSLogix 5000 by loading it on another computer, or playing with the clock in yours.

I don't know why you would bother, but there it is.

PLC CPU Speed?

Dhornberg

Member

JesperMP

Lifetime Supporting Member + Moderator

Dhornberg

Member

JesperMP

Lifetime Supporting Member + Moderator

Dhornberg

Member

rdrast

Lifetime Supporting Member

JesperMP

Lifetime Supporting Member + Moderator

Doug_Adam

Member

Peter Nachtwey

Member

Austin1970

Member

alan_505

Lifetime Supporting Member

rdrast

Lifetime Supporting Member

Similar Topics