Can cascading power to FP AI and counters cause signal drift

[MBS]

I am using the following hardware:
FP-1000 > (3) FP-TC-120
|
FP-1001 -> (3)FP-AI-111 -> (2)FP-RLY-420 -> (3)FP-CTR-500

All my AI's are 2-wire 24V loop powered 4-20mA devices (pressure transducers).
Everything is powered by one 50W power supply.
With all sensors at rest, I get 4mA on all channels; when signals are applied to transducers, some channels will drift to 7 or 8mA. Some sensors read only 15-16mA at full span, but work fine on my bench. I am seeing no drop-off in power supply voltage, so I don't think I am over-driving my supply.
Could cascading the power from the network modules to the AI and the counters be causing these problems?
It reminds me a ground loop problem, but all the sensor
s are floating and the only ground is through the AC supply to the Power supply.

[Remzi]

Hi,
I would first make sure that the module is not damaged. You can measure the resistance between Iin and COM for each channel, they should be around 100 ohms. If not, the module is not good.
Secondly, if the resistance is 100 ohms for each channel, I would just leave the FP-AI-111 on the bank (removing the relay and counter modules) and also I would leave only one pressure transducers (removing all others) and measure the current. If the reading is correct, I would connect the second one and measure the current, if the readings start deviating, you are probably experiencing ground loop problem. Make sure that you are not connecting any signal to V and C terminal of AI-111. I hope this helps.
Have a nice weekend.
Remzi A.

[Aaron_G]

Yes, cascading power may be the cause of the problems. FieldPoint IO modules are isolated from the IO stage to the inter-module communication bus. The communication bus ground is linked to the C terminal of the network module. By cascading the power from the V & C terminals of the network module to the V & C terminals of IO modules, the isolation barrier is bypassed and the IO modules no longer have floating commons. Additionally, the RS-232 port of the FP-1000 is not isolated, so it shares it's ground (linked through the C terminal) with the controlling computer. Thus, in your scenario, you have two fixed grounds, the computer and the power supply. Additionally, the input stages of the AI and CTR modules, which are normally floating with respect to the inter-mod
ule communication bus are directly tied to it (and ground) through the power cascading. Thus, as you suspect, you most likely have a ground loop from one or more of the sensors.

Regards,
Aaron

[MBS]

Thanks, Aaron...It sounds like the first thing I should do is provide a separate power supply for the network modules and comm bus to isloate them from the module's input circuitry.

Mark S.

[MBS]

Hi Remzi,
I will verify the integrity of the modules, and based on Aaron's comments, will also provide the network modules a separate power supply to eliminate the chance of a ground potential problem. My counters are working well, and don't seem to have an effect on this problem (I've removed them with no change on the sensor readings). The relays never change state during normal operation, so I believe the problem is isolated to the IO modules.
This is my first experience with Fieldpoint, and find it to be a nice system but somewhat particular about how it is configured; provisions for scaling channels individually through MAX would be a great product enhancement.

Thanks,
Mark S.