I have a CLX rack with an IF16 setup to allow connection of 16 single ended connections. Half of them are connected with sensors that have a separate supply and a 4-20mA signal back to IN# & RTN# (linked to RTN). This gives me raw data of -19660 (4mA) & 32768 (20mA). The problem is that someone has come along and added a loop powered sensor. Sending 24vdc up the + with the - connected to the IN# and the 0vdc connected to RTN# (Linked to RTN). The raw data for this channel is -20344 (4mA) and 30200 (20mA). Although at the moment I can scale this to what i want I still want to know why this should be different. I phoned AB tech support and they told me The connections were correct however I have not told them of the raw data proble as yet.

kind regards
Shaun

From the values you posted, I assume you are using the module with integer data, not floating point.
With Integers the data is a 16 bit signed interger, the possible values are -32768 to 32767.
According to my manual (Publication 1756-UM009B-EN-P - June 2003) this module is actually a 0-20mA module, per the manual for 4mA you should have -20341 counts and for 20mA 29369 counts. You only get max counts of 32767 when the current is 20.58mA or higher.

Looks like your loop powered instrument is the only one operating correctly, while it's values are off slightly from ideal, they are close enough to ignore. However, you 4-wire insturments appear to be going below 4ma and above 20mA. Are you obtaining these values by placing the instruments in "test mode" or are they actual values you have measured with a meter? I suggest you check the outputs with a meter and see what the actual output is?

You can get the analog module manual here:
http://literature.rockwellautomation.com/i...um009_-en-p.pdf

Thankyou very much for the advice. I'll check that out next shutdown.

kind regards
Shaun

If you don't already have a meter let me say that my Fluke 787 Processmeter has saved my bacon more than once in a 4-20 troubleshooting environment. Not only does it measure sources it can become a source itself.