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john_s

12-bit resolution request when using V-350

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This project requires 12-bit resolution when sampling the 4-20mA Inputs.

However, the V-350 has a 10-bit converter ...

What is a recommended way to increase it from 10-bit to 12-bits?

Perhaps a different model or some external A/D Input module...?

Any suggestions?

Regards,

John

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Both the following have 14-bit inputs.

V350-35-RA22

V350-35-TA24

The following has 12 or 14-bit selectable.

V350-35-TRA22

The trade-off with 14-bit is a slower sample rate compared to 10 or 12-bit inputs.

There are also the following external I/O modules.

IO-AI4-AO2 has 12-bit inputs

IO-AI8 has 12 or 14-bit selectable inputs.

Going to external modules adds a delay for the module communications in addition to the sample time.

If sample rate is critical, please check the datasheets for each device to determine the suitability.

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Simon,

Thanks for the suggestions. I need to stay as close to ALL-IN-ONE unit unless I expand the number of A/D inputs.

However, I am still a bit lost with the description of 4-20mA inputs as found in their docs.

For example in V-350-35-TRA22 it says...

"... Normal mode

Resolution, except 4-20mA 14-bit (16384 units)

Resolution, at 4-20mA 3277 to 16383 (13107 units)

Conversion time 100mS minimum per channel. See Note 4

Fast mode

Resolution, except 4-20mA 12-bit (4096 units)

Resolution, at 4-20mA 819 to 4095 (3277 units)..."

What are those magic numbers 3277 to 16383 (13107 units) and 819 to 4095 (3277 units) ?

Is it 12-bit or 11.5 or ...?

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They are 14 bit in normal mode and 12 bit in fast mode. Normal and Fast mode are selected in the hardware configuration.

Those magic numbers are the raw values that show up in the MI mapped to the analog input. They are the output of the A/D converter and cover the entire 0-10V or 0-20 mA range, so if you have a 4-20 mA signal you lose the bottom. This is why on a 4-20 mA signal starts at 3277 in normal mode; 0 - 3.99 mA causes the A/D to report 0 - 3276.

Run them through a Linear block to get the scaled value representing your actual data.

Joe T.

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So it's slightly less than 12-bits ..

Instead of 16/4096 it is 20/4096 [mA/digit] resolution.

I guess it's just a bit weird :-)

Does it apply to ALL units and expansion modules?

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Joe/Simon,

Fair enough - thank you.

Is there a sample program on this or older forum that shows how to read, store and display a 4-20mA input in 12-bit resolution?

I guess I ask here first before doing some convoluted search...

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There's not one that does what you specifically want, but open Visilogic and browse through the sample projects:

Help->Examples->Version 900 ....

The example programs are descriptively named and it shouldn't take long to find something close to what you're trying to do.

Joe T.

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