Strange behaviour of Analog Input Module - Siemens S7-300.

drspanda · Jul 5, 2007

Hello,

I am using 331-7KB02-0AB0 AI 2X12 bit module. Input address are PEW 200 and PEW 202...Both are configured as 4-20mA.

Now when going on-line with the PLC I found 29276 at (PEW 200) for 17.1mA input to the card.

But the manual says that we should get 0 to 27646 for 4 to 20mA.

Can anybody help me why this is so?

kamenges · Jul 5, 2007

Are you sure your measurement range modules are inserted correctly?

See Section 4.2.3 (page 14) of the PDF at this link:
http://cache.automation.siemens.com/dnl/TMzMjU1NQAA_17473828_HB/SM331_Getting_Started_Teil1_e.pdf

Keith

drspanda · Jul 5, 2007

yes it is in 'D' position.

kamenges said:
Are you sure your measurement range modules are inserted correctly? Keith

Yes I am sure that it is in pos 'D'. And I want to know how it effects the A to D conversion?

kamenges · Jul 5, 2007

I think all the scaling module does is determine how the input terminals route through module. The millivolt signals would require some amount of pre-amplification. The others would just need different resistors and terminal routing.

How did you determine that you have 17.5mA flowing through the module? Is it possible that the value is actually higher than that? Do you have any control over the value at all? For instance, what do you read with 4mA of current flow, or 8mA or 12mA? Since you are using the 'D' module position I assume the sensor you are using is a simple, 2-wire sensor. That means there is no separate supply voltage to the sensor.

I have personally only seen something like this if the scaling module was set to a voltage type input and a 4-wire current sensor was connected to the input.

Keith

JvdV · Jul 5, 2007

I have had problems in the past.

The solution for me was linking all the minussus together. For some reason the where floating around and giving wrong values on ther channels. Including the problem you discribbed.
At first you would think that linking al the zero's together wouldn't make a difference, but just try it.

If you find out why this works, pls let me know. i couldn't find out why it works.

That's for my 2 cents.....

drspanda · Jul 6, 2007

kamenges said:
How did you determine that you have 17.5mA flowing through the module? Is it possible that the value is actually higher than that?

I have determined by the use on MOTWANE multimeter and it is working ok...the same was checked by other meter as well.

kamenges said:
Do you have any control over the value at all? For instance, what do you read with 4mA of current flow, or 8mA or 12mA? Since you are using the 'D' module position

Actually I am getting this signal from vaccum transmitter 0-1100mb.

kamenges said:
I assume the sensor you are using is a simple, 2-wire sensor. That means there is no separate supply voltage to the sensor.

Yes it is 2wire Analog Input Module is not powering it and its connection diagram is below:

I hope it helps.

Jeebs · Jul 6, 2007

Hi,

According to page 19 in the manual, found at the Siemens-site, the standard 2-wire current transducer should be connected to terminals (+)2 & (-)3 for CH0.

If it is connected according to your diagram, then you should be lucky/happy your card is still working.

Jeebs

Jeebs · Jul 6, 2007

Me again.

One of the diagrams in the other forum u asked this question was very close to how it should be. Try connecting it like following diagram and tell us if it works as expected:

Jeebs

drspanda · Jul 6, 2007

Jeebs said:
If it is connected according to your diagram, then you should be lucky/happy your card is still working.

Jeebs

In fact when we first found the problem I felt the same thing....but it is wired like that and was continuing well from the installation (well in 2004).

drspanda · Jul 6, 2007

Jeebs said:
Me again.

One of the diagrams in the other forum u asked this question was very close to how it should be. Try connecting it like following diagram and tell us if it works as expected:
Jeebs

I have tried like this and found it is not working.

drspanda · Jul 6, 2007

Found one page from siemens site...

Me again

I have found a pdf for this AI module from siemens site...I have a doubt:

1. For 2DMU connection what should I do with terminals 10 (Comp+) and 11 (Comp-/MANA)?

Should I leave them not connected. OR do I need them connected to ground?

I am attaching the extract of that page.

JvdV · Jul 6, 2007

drspanda,

Connect the - connection of all the 4-20mA connections. Only a interlink, do not connect to the minus of an external power supply or earth. See what happens....... You won't be damaging anything with it..

If that doesn't help. Disconnect all other signals (leave the connection in place) then start reconnecting the other signals.

Let me know how it works out.

Jurgen

drspanda · Jul 21, 2007

After some checks...

1. checked hardware configuration and it is in 2DMU, 4-20mA and physically checked the module MR and its position is 'D'

2. I have connected terminals 10 with 11 and with M terminals...I mean all the three are connected.

3. Now I connected M+ (terminal no.4) with +ve sensor.
4. then M-(terminal no.5) with -ve sensor.

5. Module is working.

6. But for 17.2mA I am getting 32000 number in ONLINE PLC mode.

7. Why 32000??....it should be below 27648....Am I correct?

JesperMP · Jul 21, 2007

Is your common ground OK ?

drspanda · Jul 21, 2007

I think ground is ok.

JesperMP said:
Is your common ground OK ?

I think it is ok. Is there any method by which I can come to a conclusion that my ground is creating the problem? At present I am wondering about the integer data for 17.1mA exceeding 27648, which it is like that??

Whatever the other conditions may be, the amount of current that is entering into the module is converted into digital...Am I correct?

Strange behaviour of Analog Input Module - Siemens S7-300.

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