# RTU/PLC Communication

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I am working at a pump station trying to get it going, I am a civil engineer not electrical so here's my question - The PLC guy is taking all the analog signals, take for example a pressure transmitter (0 to 150 psi range) and in his program uses some 0 to 4095 range to display the signal where as the telemetry guy uses 819 to 4095 to figure his signal so when the PLC get zero pressure he gets 0 but when the RTU gets zero pressure he gets 819 in some field in his program. Anyway you can see where this could lead to a problem if the signal goes to the RTU first and then the PLC or vice versa. SCADA reads everthing from the RTU whereas the HMI at the site take everything from the PLC and things are not matching up. Someone gotta give but who's right? Where did they come up with 819 to 4095? That's my main question. Any help is appreciated. Thanks [email protected]

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#### Steve Bailey

The 4095 value comes from the 12-bit analog to digital conversion in the PLC module. There are 4096 possible combinations of ones and zeros when you have 12 bits to work with. 12-bit conversion is typical for PLC modules. A data value of 4095 represents maximum input signal level. The confusion is coming from the interpretation of a data value of zero. You're probably using a 4 - 20 mA transmitter, so 4 mA represents 0 PSI and 20 mA represents 150 PSI. However, the PLC module is probably scaled so that a 4 mA signal represents 20 percent of full scale or 819.

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#### Manoj Kalekar

0-4095 is the digital (12bit) representation of a analog system. This is how the analog signals are measured. 819 was used to set the minimum OFFSET of the RTU instrument. If the sequence to read the signal is PRESSURE INSTRUMENT -> RTU -> PLC -> then shown to MMI then PLC is to be calibrated according to the RTU (i.e 819-4095) Bye,

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#### Gordon Ho

819-4095 is used to detect under range of your transmitter calibration. General rule of thumbs to solve your problem is that : Transmitter is your source of data, and if it is hardwire connected to RTU, then PLC need to follow RTU scaling and so on.

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