SM 431 AI 16 x 13 bit capacity

Hi, I am experiencing trouble with my SM431 AI 16X13-bit module, wherein I have configured the channels to 2-wire tranducer. My sources are analog outputs from level controller, and brix controller, which I presume are "active" devices. Upon power-up, I could not get normal reading at my analog channels, all indicate an overflow which is 7FFF, I have checked the current output of my source, which read 8 mA. After reading the manuals, I learned that I have to configure the channels to 4-wire transducers for active devices. However, the manuals do not provide wiring diagrams for 4-wire setting of this module. Also, does this mean that the capacity of my module for reading 4-wire transducers also reduces? Originally, I have 16 AI slots available, does this count only for overall 2-wire configuration or not? Please help me with this. Thanks.

page 189 S7-400 system module manual

Max. permissible common mode voltage:
SM432 AI 16x13
Between the channels or the reference potentials of the connected sensors and central ground point: 2 VDC/AC

2 volts isn't much. I suspect you have a common mode ground loop problem.

The ground loops referred to in the previous thread can drive a signal off scale, whether it's up scale or down scale depends on the polarity of the ground loop current and difference in ground potential.

If you have a 4-20mA loop simulator, I'd suggest connecting it across the analog input's (+) and (-) terminals and seeing what value you get. If you get a good reading with a battery powered loop simulator, then it says the the AI is functioning. I suspect a floating, battery powered simulator will work fine.

You might try connecting only one of the field devices and see how it works by itself. It is not uncommon for an AI card to tolerate a small amount of common mode voltage from one source (like 2V), but the addition of a 2nd source will add its common mode and will drive all the signals off scale. You might get by with one device that works and one device that needs a loop isolator.

>I learned that I have to configure the channels to 4-wire transducers for active devices. However, the manuals do not provide wiring diagrams for 4-wire setting of this module.

I suspect that the difference is that '4 wire' mode does not need loop supply voltage, because it's an active output from the field device. '2 wire' mode probably infers 'loop powered' by the analog input, 4 wire probably infers 'no loop power needed', but that's speculation on my part (the manual's too thick to read at 11PM).

The 3rd and 4th wires refer to the power supply for the field device.

This could be a configuration error, I'm not a Siemens guy, but the symptom is typical of a common mode ground problem. If it ends up that you have a common mode ground loop problem, the solution is a loop isolator module, unless you can bear the expense and effort of forcing the grounds at both locations to an equal potential.

Thanks for your reply. I would first try the 4-wire configuration, it is indicated in the manual that I might have to remove the module from the rack to be able to change the configuration. By the way, the 4-20 mA loop simulator is a good idea, I might actually try it. I would check the grounding, but I'm quite sure that these are grounded at both locations.

Regine said:

but I'm quite sure that these are grounded at both locations.

Click to expand...

That's what causes ground loops and the common mode voltages that drive signals off-scale - when the grounds are not at the same potential, current has to flow, and then there's a ground loop.

Keep us posted.

Can you please clarify what you mean by "when the grounds are not at the same potential", does this mean that the grounding at both sides should likely be at one location?