Speed setpoint INC and Dec

[pauly]

I have a speed setpoint for a drive coming from an HMI, and there are INC and Dec buttons on a desk to increase and decrease this setpoint.
My question is, if I have an INC/DEC function in my PLC acting on my HMI value, will it be overwritten from the HMI itself?
At the moment I have no hardware available to check. I am using TIA Portal

 

[goghie]

It depends of program itself, it can be done in both ways.

 

[BryanG]

Generally the HMI reads data cyclically from a data source. You can usually set how often the HMI reads the data, though the rate is also affected by the speed of communication between the HMI and PLC and the quantity of values the HMI currently needs to read.

The HMI often writes single values to the data source individually, so when you change the data at the HMI, it is written straightaway to the PLC. Most HMIs that I have used have one box where you say what register to display and write. Redlion HMIs allow you to split it in to two, so you can display one register, but write to a different one.

Obviously there is a timing issue where if you change the value at the PLC and HMI at the same time you won't know which value will be used. But the HMI will catch up on the next read cycle and it will display the current value in the PLC.

This is written with all the usual caveats, in the end it depends on how your particular HMI works.

 

[parky]

Most INC/DEC from an HMI I have seen uses while button pressed (i.e. on while pressed) & in the PLC they use the bits plus a time contact on a oneshot to increment or decrement the value, generally speaking if you use the scan of the PLC this could increment or decrement the value too fast, the other way is to use the HMI one shot function (some have them) but again this can be tricky as the oneshot might be on for more than one scan, communications delay means it may increment/decrement at different rates.

 

[drbitboy]

The answer to the OP's question hinges on which device is the ultimate source for the speed setpoint value that goes to the drive. Is it in the PLC, and the HMI commands the PLC when to increment the value (see example below; debounce if needed, etc., are not shown)? Or is the setpoint value maintained and incremented in the HMI, which sends the setpoint value to the drive (perhaps via the PLC)?


So without more information, OP's query cannot be definitively answered yet.