Allen Bradley PLC SCP help

Aug 04, 2005 4 Replies
B

We have an Allen Bradley PLC programmed by others. I have two analogue inputs connected and their inputs are programmed to a SCP Scale with parameters block. The two 4 to 20 mA inputs are from gas monitors ant the range of the monitors is 0 to 90 parts per billion. Both inputs are the same and perform the same functions. However, both SCP blocks are scaled differently and ask if one of them is incorrect.



4 to 20 mA input = 0 to 90 PPB. Block 1 is programmed as Input min = 3277 Input max = 16384 Scaled min = 0 Scaled max =90

Block 2 is programmed as Input min = 3277 Input max = 16384 Scaled min = 0 Scaled max =900



Appreciate any advice. TIA


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P

Gas monitoring isn't my field, but could it be that the system designer wanted the first sensor to give an output with a resolution of whole units but the second one with an effective resolution of 0.1 units. So he scaled the max to convert to a figure of 900 rather than 90, on the second.

Provided that the rest of the system was set up with that in mind, no problem.

ISTR one system that was actually sold as a "low resolution" system with an (expensive) upgrade to "high resolution", per channel. The upgrade came in a big box (to impress the customer that he was getting value for money) but actually consisted of a quick firmware tweak as above.

HTH Sue

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S

I will bet that Block 2 is a typo.. Does block 1 work correctly?

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B

Yes it works correctly, I think you are right that it is a mistake.

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S

It is easy to make mistakes when programing PLC's. I check and recheck every key stroke 3 times before walking away from a program.

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