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Old December 11th, 2003, 03:09 PM   #1

GTX is offline
Join Date: Dec 2003
Posts: 36
AB Analog Input Help


I have a Micrologix 1500 with an analog Input module hooked up to a variable frequency drive so i can monitor the RPM it is putting out. I have the VFD Configured to ouput 0-10volts. The problem I am having is that the input i am getting through the analog input card is a negative number. For example, right now i have the VFD setup to output 40 Hz which is 6.67 volts. The problem is my Analog Input displays this 6.67 volts as -500.

Here is how i have Analog input module setup:

- the channel is enabled
- data format is Engineering Units
- data range 0-10 volts
- I have the input from the VFD wired into the voltage in on the module

Can anybody tell me why i am getting a negative input ? How do i configure the input so i will get a number like 7000 when the VFD is outputting 7volts ?

Sorry for the newbie question but this is the first time i have used analog input and output modules

Thanks for any help

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Old December 11th, 2003, 03:22 PM   #2
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firebrd10 is offline
Join Date: Sep 2003
Location: Kentucky
Posts: 61
This may be pretty basic but have you tried swapping the leads on your analog input.

Are you using some type of scalling instruction.
If so what variables are you using.

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Old December 14th, 2003, 02:37 PM   #3
Tom Jenkins
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Location: Milwaukee, WI
Posts: 5,967
I haven't used the voltage inputs on the 1500, but the 4-20 mA inputs on the 1500 are a little quirky. First of all, under "I/O Configuration / Advanced Configuration" there are a lot of options for data format - make sure the one you pick matches the one your logic is based on. Second, teh cards, althoughthey are nominally 16 bit, have a lot of drift in zero and span. I always make sure I use a simulator to get the data at known actual min and max range, and use that info in conjunction with the SCP funtion to convert to engineering units, and then I use the engineering units in my logic.

Abnd, finally, you neglected a key piece of info - what voltage and what polarity did you catually measure at the input card terminals? Never discount the obviuos solution!
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