Interfacing Of Plc And Hmi

[Wsandeep]

Hi
Please help me to understand how alarm tagging to be done so that alrm desplay on HMI as soon as alarm condition declaired by PLC

 

[Singapore_Mats]

plc monitor all inputs, and when input goes from off to on, or on to off, the change is sent by modbus or other protocol to the hmi.

and the hmi then does what you programmed it to do.

 

[rdrast]

It is impossible to display an alarm on any HMI in existance "as soon as the alarm condition declaired by PLC".

An alarm condition in a PLC must remain in an alarm state for at least one complete polling cycle of the communications link between an HMI and a PLC.

Typical HMI's monitor a "Tag", which is connected to a data server, which is connected to a PLC. The Data Server only polls the PLC for information periodically. Even if you try to poll every millisecond, you won't be getting an accurate value for every point in the PLC every millisecond. Commonly, HMI alarming is polled on a 250, 500, or 1000 millisecond basis.

If you know that your data server is polling at 250msec or faster, than to assure that an HMI can 'see' an alarm condition, you must make sure the alarm condition is maintained for AT LEAST 250msec... and preferably, add 10% to that, so you want the alarm 'Bit' to stay 'Active' for at least 275msec.


(Caveat: If the HMI is PART of the PLC, as some smaller 'integrated HMI/PLC's are, it just might be possible)

Hi
Please help me to understand how alarm tagging to be done so that alrm desplay on HMI as soon as alarm condition declaired by PLC

 

[Tom Jenkins]

I recommend making the alarms in the PLC "latched" - that is, when the alarm condition occurs an internal coil is turned ON and a holding contact will keep the alarm coil ON until the alarm is cleared from the HMI. Then you don't have to worry about polling cycles

 

[wildswing]

I recommend making the alarms in the PLC "latched" - that is, when the alarm condition occurs an internal coil is turned ON and a holding contact will keep the alarm coil ON until the alarm is cleared from the HMI. Then you don't have to worry about polling cycles

That's how we do it. Seal (or latch) the alarm bit that's read by the HMI. Break the seal (unlatch) with operator acknowlwdge button...

 

[Wsandeep]

thanks

Thanks to all of u for helping me to understand the problem.

Now another problem, how can i see the actual motor rpm on pc with the help of siemens s-7-300 plc ,simatic VC drive,wincc scada and communication by profibus.

 

[Singapore_Mats]

either your motor controller can provide info about rpm, or the motor have some sort of feedback, tachometer or similiar.