Serial commmunication forS7-300

[panou]

Hello
I want to connect a load cell transmitter to a Simens S7-300 CPU in order to collect more accurate data and have the totalizer in my PLC code and not done by the weighing transmitter itselff. How can this be relaised ? Of course i will need serial cable and progrmming. Is there any restriction to the cable length? Thanks in advance for your time.

 

[Rheinhardt]

Siemens has a Serial communication card available for the S7-300..

Goto

http://support.automation.siemens.com

You can choose if you want to use RS232 or RS485,,ect.

Hope this helps..

RP

 

[Innovia]

Mmm, I don't thinck Rheinhardt has completely understood the question.

I suppose you want to connect the loadcells directly to you PLC. As you know the principle of a weighing loadcell is based on a resistance bridge and measurement is given in mV/V. This means that for a certain supply voltage to the load, the bridge will give a certain mV signal. First thing you need to be sure of is that the supply voltage is either very stable (beware of voltage drop when using long cables) or that you will have to include a measurement of the supply voltage. Most of the cases the supply voltage is between 10 and 20 Vdc. A typical loadcell will with such a supply give a signal of about 20 to 50 mV! This will have to go to an analogue input of the PLC which is accurate enough!
Temperature variations will also influence the measurement.

My opinion : by an of-the-shelf transmitter (calibrated) which gives you a 0-10Vdc or 4-20 mA signal and read this into a standard analogue input.
Some transmitters do have a serial interface (RS232 for single channel or RS485/422 for multiple channel). The advantage is that the measurement comes in a digital way (accuracy) but it will be slower as communication takes up time.

Good luck.

 

[RolfB]

For S7-300 you should look at 'SIWAREX'
on 'Simatic Website'

http://support.automation.siemens.com/WW/view/en/7787659

Cheers
Rolf