RTU/PLC Communication

  • Thread starter Adriana Erickson
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Adriana Erickson

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]

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.

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,
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.
Adriana, your "PLC guy" should be able to explain where the numbers come from and all conversions from raw data to engineering units. Try basic algebra..2 equations, 2 unknows, Y = MX + B, Y= engineering units, M is slope, PSi per ADC count, and X is the raw ADC count number, and B is the intercept. The numbers come from the hardware and basic ohms law. Normally a 250 ohm resistor (R) converts the incoming current (I) to volts E, E = I times R, simple equation for a civil engineer. The 4 to 20 ma pressure transmitter signal produces a 1 to 5 volt signal for the PLC input module hardware.The hardware of the PLC needs to be understood as to how this analog signal is processed internally. It appears that internal hardware scaling has the maximum output of the 12 bit ADC, 4095, as full scale for the 20 ma signal.With a linear relationship, the 1 volt signal from the 4ma signal is 20% of full ADC, 20% times 4095 = 819 ADC counts. This is a bit (no pun intended) confusing since 4095 normally represents a full 10 volt signal, vut the current input module represents full scale of 20ma as full scale of 4095. The 4095 represents a 12 bit ADC, 2 to the 12th power minus 1 equals 4095. Simple per cent span calculations for this linear relationship should enable conversion from raw ADC counts to engineering units of pressure. You have a span of 16 ma(20-4), a span of 4 volts (5-1), and a span of 3176 counts (4095 - 819), and a span of 150 psi (150-0). At 50% of span, 12ma (50 % of 16 ma added to 4ma), 3 volts (50% of 4 volts added to 1 volt), and 2407 ADC counts (50% of 3176 counts, 1588, added to 819), should yield an engineering unit value of 75 psi. Why not have your "PLC guy" convert the raw ADC counts to engineering units in the PLC and transmit pressure values? Play with the numbers. When you do the Y + MX + B calculations, B is a negative number. Just for fun, if you have a 14bit analog to digital convertor, the 2 to the 14th power minus 1 is the maximun ADC count value. But again it depends on the hardware, since some internal scaling inthe input modules with a 14bit ADC, actually throw away the two lower order bits, and only keep 12, shifted left justified. Back to basics..there should not be any mystery as to where the numbers come from. Now for some real fun with numbers. Take the Golden Mean number found in art and nature and mathematics. The numer squared is itself plus one. The reciprocal of the number is itself minus one. Again two equations, two unknowns, to solve use the quadratic equation from highschool. The number is the square root of 5, plus 1, all divided by 2. The Golden Mean number, phi, is 1.61803398875. Great self check for a calculator. Add one for phi squared. Subtract one for the reciprocal of phi.
Thank you for that excellent response to show the relationships between these sometimes overlooked simple math steps. I've heard many people try to explain this in many different ways, and this is by far the best explanation I've seen in a long time. Thank you. Now....this phi you speak of....:)