PLC - FIXED SCAN PROGRAM

Hi,
I am newby in plc programming. I have doubts about influence on Fixed scan program on CPU work. I have Melsec FX5U PLC controller and I have recently experimented with fixed scan program. I have created FC block and in case of execution function in Main Program (without interrupt), PLC cycle time was about 3 ms (max. cycle time - about 5ms).

The same function I have move from Main Program to Fixed Scan. In case of intterrupt 5 and 6ms the cycle time was as in the first case - about 3 ms (max. cycle time - about 5ms). In case of interrupt 4ms or less the cycle time was about 29ms and max. cycle time - 70ms.
It's quite consistent, but I have doubt if CPU will work properly in case of fixed scan program with interrupt cycle time lower than necessary time to execute the same program without interrupt.

Thank you in advance for answers!
 
I've checked the Melsec FX5 Programming Manual and there is no indication of fixed scan capability.
What is interesting is the ' interrupt' doesn't work like other PLC's that have 'timed interrupts'.

It's not clear what you are trying to achieve - since fixed scan and interrupt capability are mutually exclusive [they don't work together very well].

Perhaps we can check whether you have a different manual or the controller you have does have fixed scan capability.
 
I've checked the Melsec FX5 Programming Manual and there is no indication of fixed scan capability.
What is interesting is the ' interrupt' doesn't work like other PLC's that have 'timed interrupts'.

It's not clear what you are trying to achieve - since fixed scan and interrupt capability are mutually exclusive [they don't work together very well].

Perhaps we can check whether you have a different manual or the controller you have does have fixed scan capability.
Interrupt Pointer is necessary to define "fixed scan time" (I28-I31). Could you share with me how does it work in other PLC's ?
It is very important for me, because I would like to implement control algorithm for servo drive velocity and position loop (I use torque mode in servoamplifier) and in that case the smaller fixed cycle time the better.

The FX5U have watchdof which generate error if cycle time would exceeded defined time (default - 2000ms). I have conducted some experiments and in case of cycle time about 90ms, if I add some code to project the watchdod have generated error. To me it indicates that I have right that in case of small fixed cycle time, the fixed scan program executes properly, but at the cost of total cycle time.
Please correct me if I'm wrong.
 
Having checked both the FX5U manual and Siemens s7/400 manual, neither have 'fixed scan' capability, indeed the only time I've seen fixed scan used in the past is when the PLC had no interupt capabilty but had PID functionality which needed accurate update times.

Both Melsec FX5 and Siemens s7/400 interupts operate in a similar manner, except the FX5U has a disability function, and the Siemens you specify timed interupts [500 μs to 60000 ms ] to suit your application - particularly fast PID's.
This makes the idea of fixedscan timing a thing of the past.
 
Having checked both the FX5U manual and Siemens s7/400 manual, neither have 'fixed scan' capability, indeed the only time I've seen fixed scan used in the past is when the PLC had no interupt capabilty but had PID functionality which needed accurate update times.

Both Melsec FX5 and Siemens s7/400 interupts operate in a similar manner, except the FX5U has a disability function, and the Siemens you specify timed interupts [500 μs to 60000 ms ] to suit your application - particularly fast PID's.
This makes the idea of fixedscan timing a thing of the past.

Fixed scan is a name of program in GX Works 3 which work with timed interrupts (with resolution of 1ms). Do you know if I have defined time interrupt with time lower than time necessary to execute the same code in Main Scan, the CPU will work properly?
In other words, is it risk of executing "Fixed Scan" program slower than time defined in interrupt?
 
GX Works 3 gives no indication that I can see of ' Fixed scan'; it appears only to confuse.

have defined time interrupt with time lower than time necessary to execute ..
I cannot see where you could shorten the execution of any code without essentially aborting your application.
Mutsubishi must mean something different.

Perhaps you can work through what Mutsubishi have [in mind] and let us know the outcome !
 
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