Alan said...
"Terry I disagree. A HMI cannot change the state of an input.
The input is either off or on dependant on what is happening in the real world. You can use the HMI to decide whether to use the input value or to use another value but you can't change what the input state is. (This thread is based on a ML1200)..."
Alan...
I did not suggest that an HMI could change the state of an Input... I said it could be used to disable the input.
Now, having cleared that up, I will go on to say that, as far as the program is concerned, an HMI can certainly be used to change (reverse) the state of an input.
BTW, the following applies to
ANY PLC!
INPUT CTRL-BIT
---| |------|/|----+----( ) Input Proxy
|
INPUT CTRL-BIT |
---|/|------| |----+
.
Input Proxy is used to represent the Input in the program.
The HMI controls the CTRL-BIT.
If CTRL-BIT is OFF then Input Proxy is controlled by the --| |-- Input.
Input ON = Proxy ON, and Input OFF = Proxy OFF
If CTRL-BIT is ON then Input Proxy is controlled by the --|/|-- Input.
Input OFF = Proxy ON, and Input ON = Proxy OFF
If the field device is Normally
OPENED and you want a "Positive" Indication in your program then leave CTRL-BIT OFF.
If the field device is Normally
CLOSED and you want a "Positive" Indication in your program then turn CTRL-BIT ON.
There ain't no rule that says you can only use actual Input Status in the program!
This can be a mighty handy tool for allowing for the possibility of having to replace a Normally OPENED device with a Normally CLOSED device(*), or vice-versa.
(*) Because it happens to be the only replacement available!
And because the Input is represented by the Proxy, there is no need to rework the code!
A "Sensor-Type" page on an HMI would allow Maintenance folks to change the "expected sensor-type" based on what they had to install... without having to go into the program.
Alan also said...
...As far as using the HMI to disable an input then that is also impossible short of faulting the processor. Even with an SLC all you can do with a HMI is disable a whole card not an individual input."
Disabling an Input is a piece-of-cake!
This also applies to
ANY PLC!
INPUT CTRL-BIT
---| |------|/|---------( ) Input Proxy
.
In this case...
Again, Input Proxy is used to represent the Input in the program.
...If the HMI-controlled CTRL-BIT is OFF then the Input is Enabled.
...If the HMI-controlled CTRL-BIT is ON then the Input is Disabled.
If CTRL-BIT is ON, then, for all practical purposes the Input is disabled!
I have several instances of this in my main process... it has prevented a lot of late-night phone calls!
This PLC-stuff is nothing but playing logic games! All it takes is IMAGINATION!
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