Siemens PLC analog module usage questions

S7-200 analog input module (EM231,EM235) addressing


Each analog expansion module, sorted by extension modules in the order, in which the analog input and output according to different sorting, respectively. Analog data format is one word, so the address must begin with an even number of bytes, precision 12-bit; analog values with the value 0-32000. Input format: AIW[a starting byte address] AIW0 output formats: AQW[a starting byte address] AQW0 each analog input module, module address is fixed in the order, the order to the rear. Example:: AIW0,AIW2,AIW4……, AQW0,AQW2 … ….


At least two channels per analog expansion module, even if the first module has only one output (AQW0 EM235 has only an analog output), the second analog output modules address should also AQW4 to start addressing, and so on.

2. connect the sensor to S7-200 analog input module (EM231,EM235) what are the points for attention?


Analog input module can set the dip switches to different ways of measuring (voltage and current). Module switch settings apply to the entire module, a module can only be set to a measuring range and switch to take effect only after the electricity back on. Input at the same time can only be set to a range and the format, that is, the same input range and resolution.


EM235 is the most commonly used analog expansion module, it implements 4channel analog input and 1 analog output. EM235 analog expansion module wiring method for voltage signals, by positive and negative direct access to X+ and X-; for the current signal, RX and X+ after the short access current input signal + side; channel X+ and X-are not connected sensor shorted.


Note: in order to avoid the commonmode voltage, m end with all signals end to be connected, not connected sensor channels to be shorted. When analog input PLC received a change in very unstable when, for one reason: you may use a self-powered sensor power supply or isolated, both power supply not connected to each other, so the resulting bounce of a high commonmode voltage, affects the analog input value. The second reason: analog input module wiring may be too long or bad insulation. Solution: 1. connect sensor input negative terminal with modules on the public m-side to compensate such fluctuations. (Note: prior to determine if this is the only connection between the two power supplies. If a connection already exists, when added public connection may have an extra compensating current.


When the analog input PLC receives a signal change is slow, this could be you use filters, can be reduced by filtering samples, or cancel the analog filtering solution.

3. EM235 can be used for heat resistance temperature measurement problem?


EM235 module is not used in connection with the thermal resistance measurement temperature, barely used risk failure.

4. EM235 input to calibrate:


Analog input module input calibration before use. Enter calibration prior to shipment, if the OFFSET and GAIN potentiometer is readjusted input calibration is required. The steps are as follows:


A, cut off the power supply module, select the desired input range.


B, connect CPU and power supply modules, the module stability for 15 minutes.


C, a transmitter, a voltage source or current source, the zero value signal applied to an input terminal.


D, read the appropriate input channel measurement value in the CPU.


E and adjust the OFFSET (offset) potentiometers until it reads zero, or need numeric data values.


F, received a full scale signal input terminals, read out the value sent to the CPU.


G, adjust GAIN (gain) potentiometers until reads 32000 or need numeric data values.


H and, if necessary, repeat the offset and gain calibration process.

5. the conversion of the analog values, and the a/d conversion:


Assuming that analog standard signals are A0-Am (such as 4-20mA), after a/d conversion values for D0-Dm (such as 6400-32000), analog standard signals are A,A/D after the corresponding value of d, because it is a linear, function A=f (d) can be expressed as a mathematical equation:


A=(D-D0)×(Am-A0)/(Dm-D0)+A0。


According to the formula d value can be easy to calculate a value. This equation inverse transform, D=f draw function (a) mathematical equation can be expressed as:


D=(A-A0)×(Dm-D0)/(Am-A0)+D0。


To quote an example, a case study of S7-200 and 4-20mA, a/d conversion, we get a value of 6400-32000, the A0=4,Am=20,D0=6400,Dm=32000, into the equation is obtained:


A=(D-6400)×(20-4)/(32000-6400)+4


Hypothesis corresponds to the analog and AIW0, when the value of AIW0 is 12800, the corresponding analog signal is 6400×16/25600+4=8mA.


As another example, a temperature sensor,10-60 corresponds to 4-20mA, and t represents the temperature value, AIW0 PLC analog sampled values according to type directly into the draw:


T=70×(AIW0-6400)/25600-10


Can t displayed temperature value.


Conversion of the analog values, and the a/d conversion more difficult to understand, that paragraph several times, combined with the examples, you will understand. To make it easy for you to understand, let me give you an example:


A pressure transmitter, when the pressure reaches full scale 5MPa, pressure transmitter 20mA,AIW0 is the output current value is 32000. Visible/MA A/D that corresponds to the value of 32000/20, when measuring the pressure 0.1MPa, pressure transmitter should be current as of 4mA,A/D (32000/20) X4=6400. It follows that the AIW0 value to the actual pressure value (units KPa) is calculated as:


The value of VW0 = (value of AIW06400) (5000-100)/(32000-6400) +100 (units: KPa)

Programming examples


Instance demo that you can form a small analog programming. CPU in this instance is CPU222, with only an analog extension EM235 module, the first channel of the module to connect a 4-20mA transmitter output temperatures display instrument, the instrument‘s range is set to 0-100, or 0 degrees when output 4mA,100 20mA output. Platinum resistance temperature display instrument input connect a 220 Ohm variable potentiometer, simple programming are as follows:


Temperature display value = (AIW0-6400)/256


Compile and run the program, the watcher, the VW30 temperature value is displayed, control instrument display values are the same.