Siemens S7/TIA: Don't understand the digital value for 4-20mA...

[Mas01]

On the laser displacement sensor now connected to my PLC, the manual says 4mA=643 and 20mA=64,877.
However, I checked with a Fluke, injecting milliamps and 4mA was 0 and 20mA was 27,648.
So how can 20mA be 64,877 for this sensor?
Image to follow...

 

[I_Automation]

The values on your datasheet pic are internal in the sensor.


They don't know which PLC you are using, some are 12 bit and others 16 or 32 bit.


I have seen 20mA read 4098, 12,000, and 32766 in different PLC's



The PLC manual for the analog input should say what that PLC has for 4mA and 20mA, or hook up a calibrator when online and see what it actually reports.

 

[Mas01]

The values on your datasheet pic are internal in the sensor.


They don't know which PLC you are using, some are 12 bit and others 16 or 32 bit.


I have seen 20mA read 4098, 12,000, and 32766 in different PLC's



The PLC manual for the analog input should say what that PLC has for 4mA and 20mA, or hook up a calibrator when online and see what it actually reports.

Ah understood thanks.
I recognise that magic number of 4098 - it's used on Mitsubishi PLCs.
I'm using S7-1200.
Cheers

 

[ojz0r]

The 64k value is the internal value of the sensor in 16 bits (65535).
Siemens measures up to 27648. That value should be present when you hook the sensor up to the analog input (if its at maximum).

 

[Mas01]

The 64k value is the internal value of the sensor in 16 bits (65535).
Siemens measures up to 27648. That value should be present when you hook the sensor up to the analog input (if its at maximum).

Very helpful, ojz0r, tack så mycket.

 

[RVaughan]

The 64k value is the internal value of the sensor in 16 bits (65535).
Siemens measures up to 27648. That value should be present when you hook the sensor up to the analog input (if its at maximum).

At the risk of Too Much Information...

27648 represents 100% of the process value when measuring voltage, current, or resistance (in your case, 20ma). The card will return a value up to about 111.8% of the max, up to 32787. Diagnostic reporting is generally available for values that exceed this limit (Overflow).

Unipolar voltage/current/resistance measurements have a floor (zero percent) of zero, while bipolar measurements have a floor (negative 100%) of -27648.

Unipolar and bipolar are also referred to as Symmetrical and Asymmetrical, respectively, in Siemens documentation.