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SIMATIC S7-1200

SM 1234 analog input/output module

Thread Starter: mitasha   Started: 5/13/2012 6:51 PM   Replies: 10

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  5/13/2012, 6:51 PM
mitasha
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Can sm 1234 analogue input module be used as a loop power source to power a two wire sensor like sitrans probe lu.
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  5/13/2012, 7:51 PM
mitasha
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i used this to connect to the sitrans probe lu and was getting currents of up to 30ma instead of 4-20ma
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  5/13/2012, 8:57 PM
huggy_d1
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As long as the SM1234 has external 24VDC power supplied and you just run the shielded twisted pair from the AI +/- to the Sitrans Probe LU terminals +/-, it should work just fine.

Any other wires may cause problems. 

Be sure to configure the analog input channel for current and downloading the hardware configuration to the PLC before connecting up the sensor.
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  5/13/2012, 10:04 PM
Gorp
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mitasha wrote:

i used this to connect to the sitrans probe lu and was getting currents of up to 30ma instead of 4-20ma



The Sitrans probe LU is a two wire device.
What you show is a four wire(0-10V?) device.....Rnf:lblSmileyTooltip_ think
If it is a two wire 4-20mA device you are using, you should connect it like this:

    L+(+24VDC) to the + of the device
    M(0V) to AI- of the module
    the - of the device to AI+ of the module
Always read the manuals first.
Then use the "information system" also known as "help".
If that does not bring you further, ask a question but... give all the details!
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  5/14/2012, 8:29 AM
Prats
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SM 1234 can be used for the following inputs.

1) +/- 10V
2) 0-20 mA
As you have mentioned 4-20 mA kindly ensure the scaling is done for the right count.
Regards,
Prats
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  5/14/2012, 1:05 PM
koenb
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Have a look at this post , it show’s an wiringexample

sink type or source type

koenb
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  5/14/2012, 4:48 PM
mitasha
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meant to post this picture. was getting about 30 ma from sitrans probe lu and it does not drop. When i connect the A+ and A- to the probe directly even after powering the analogue module externally the probe does not come on
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  5/14/2012, 5:28 PM
huggy_d1
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When you downloaded the plc program, you configured your analog inputs for current before connecting your probe wiring?  If not, then the device will not have any power.
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  5/14/2012, 6:20 PM
Gorp
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huggy_d1 wrote:
When you downloaded the plc program, you configured your analog inputs for current before connecting your probe wiring?  If not, then the device will not have any power.

And.... there would be no 30mA flowing from the sensor to the input as mitasha writes!

@mitasha,
Your connection diagram looks ok.
I guess you have to test both your sensor and your module to find out which of the two is not functioning properly.
But to do that you need some (variable) resistors.
If you don't have those, is it's no use to explain how to do the tests....
Always read the manuals first.
Then use the "information system" also known as "help".
If that does not bring you further, ask a question but... give all the details!
No need to thank, no need to rate.
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  5/14/2012, 6:57 PM
huggy_d1
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mitasha wrote:

meant to post this picture. was getting about 30 ma from sitrans probe lu and it does not drop. When i connect the A+ and A- to the probe directly even after powering the analogue module externally the probe does not come on

In this wiring configuration, you basically put your sensor in a voltage divider network with the sensor's internal resistance in series with the analog input module's internal resistance, dropping a consistent amount of voltage across both regardless of the sensor feedback.

Once you get the PLC hardware configuration downloaded and the plc started, what voltage is present (without wires attached) to the analog input +  & - terminals?  If it is at least the same as the 24V supply, your probe should work provided your wiring polarity matches.

Connect the wires to the analog input module, then at the probe, measure the voltage at the leads.  Ensure the + goes to the right terminals and - to the left terminal (according to the probe manual they are labeled - & + (left to right).

The probe should power on with a straight 24V DC power supply and allow configuration to tell it you will use loop (4-20mA) configuration.  You may need to preconfigure the probe for it to function properly.  Read the probe manual for that as it is beyond the scope of this forum.

With a little more testing & configuration, your probe should be operational very quickly.

I'm glad I took the time to really read your question thoroughly.  I might have given bad advice otherwise.
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