When an SV higher than 9999 is required, two counters can be combined as shown in the following example. In this case, two CNT instructions are combined to make a BCD counter with an SV of 20,000.
Showing posts with label Programmable logic controller. Show all posts
Showing posts with label Programmable logic controller. Show all posts
Wednesday, 14 January 2015
Friday, 2 January 2015
Response Time Concerns PLC
Now that we know about response time, here's what it really means to the application. The PLC can only see an input turn on/off when it's looking. In other words, it only looks at its inputs during the check input status part of the scan.
In the diagram, input 1 is not seen until scan 2. This is because when input 1 turned on, scan 1 had already finished looking at the inputs.
Input 2 is not seen until scan 3. This is also because when the input turned on scan 2 had already finished looking at the inputs.
Input 3 is never seen. This is because when scan 3 was looking at the inputs, signal 3 was not on yet. It turns off before scan 4 looks at the inputs. Therefore signal 3 is never seen by the plc.
To avoid this we say that the input should be on for at least 1 input delay time + one scan time.
But what if it was not possible for the input to be on this long? Then the plc doesn't see the input turn on. Therefore it becomes a paper weight! Not true... of course there must be a way to get around this. Actually there are 2 ways.
Pulse stretch function. This function extends the length of the input signal until the plc looks at the inputs during the next scan.( i.e. it stretches the duration of the pulse.)
Interrupt function. This function interrupts the scan to process a special routine that you have written. i.e. As soon as the input turns on, regardless of where the scan currently is, the plc immediately stops what its doing and executes an interrupt routine. (A routine can be thought of as a mini program outside of the main program.) After its done executing the interrupt routine, it goes back to the point it left off at and continues on with the normal scan process.
Now let's consider the longest time for an output to actually turn on. Let's assume that when a switch turns on we need to turn on a load connected to the plc output.
The diagram below shows the longest delay (worst case because the input is not seen until scan 2) for the output to turn on after the input has turned on.
The maximum delay is thus 2 scan cycles - 1 input delay time.
In the diagram, input 1 is not seen until scan 2. This is because when input 1 turned on, scan 1 had already finished looking at the inputs.
Input 2 is not seen until scan 3. This is also because when the input turned on scan 2 had already finished looking at the inputs.
Input 3 is never seen. This is because when scan 3 was looking at the inputs, signal 3 was not on yet. It turns off before scan 4 looks at the inputs. Therefore signal 3 is never seen by the plc.
To avoid this we say that the input should be on for at least 1 input delay time + one scan time.
But what if it was not possible for the input to be on this long? Then the plc doesn't see the input turn on. Therefore it becomes a paper weight! Not true... of course there must be a way to get around this. Actually there are 2 ways.
Pulse stretch function. This function extends the length of the input signal until the plc looks at the inputs during the next scan.( i.e. it stretches the duration of the pulse.)
Interrupt function. This function interrupts the scan to process a special routine that you have written. i.e. As soon as the input turns on, regardless of where the scan currently is, the plc immediately stops what its doing and executes an interrupt routine. (A routine can be thought of as a mini program outside of the main program.) After its done executing the interrupt routine, it goes back to the point it left off at and continues on with the normal scan process.Now let's consider the longest time for an output to actually turn on. Let's assume that when a switch turns on we need to turn on a load connected to the plc output.
The diagram below shows the longest delay (worst case because the input is not seen until scan 2) for the output to turn on after the input has turned on.
The maximum delay is thus 2 scan cycles - 1 input delay time.
What is a PLC?
A
PLC (i.e. Programmable Logic Controller) is a device that was invented
to replace the necessary sequential relay circuits for machine control.
The PLC works by looking at its inputs and depending upon their state,
turning on/off its outputs. The user enters a program, usually via software,
that gives the desired results.
PLCs
are used in many "real world" applications. If there is industry
present, chances are good that there is a plc present. If you are involved
in machining, packaging, material handling, automated assembly or countless
other industries you are probably already using them. If you are not,
you are wasting money and time. Almost any application that needs some
type of electrical control has a need for a plc.
For
example, let's assume that when a switch turns on we want to turn a
solenoid on for 5 seconds and then turn it off regardless of how long
the switch is on for. We can do this with a simple external timer. But
what if the process included 10 switches and solenoids? We would need
10 external timers. What if the process also needed to count how many
times the switches individually turned on? We need a lot of external
counters.
As
you can see the bigger the process the more of a need we have for a
PLC. We can simply program the PLC to count its inputs and turn the
solenoids on for the specified time.
This
site gives you enough information to be able to write programs far more
complicated than the simple one above. We will take a look at what is
considered to be the "top 20" plc instructions. It can be
safely estimated that with a firm understanding of these instructions
one can solve more than 80% of the applications in existence.
That's right, more than 80%! Of course we'll learn more than just these instructions to help you solve almost ALL your potential plc applications.
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