Scalling for analog input module - SLC 5/04

ckchew666

Member
Join Date
Aug 2003
Location
Malaysia
Posts
591
Guys,

I'm checking on some of the analog input at the SLC 5/04, I couldn't understand some input (min & max) & scaled input (min & max) settings.

1) For example, I tried to inject 4-20 mA into one of the analog channels (completely remove all the 2 cables at the channel), I managed to get the below reading at RSLogix 500:

Inject (mA) Value

4 3271
8 6545
12 9818
16 13094
20 16368

But at the SCP instruction

Input = I:6.3 (3150)
Input Min. = N7:56 (3175)
Input Max. = N7:57 (9007)
Scaled Min. = N7:66 (0)
Scaled Max = N7:67 (5000)
Output = N7:174 (-21)

a) I taught the Input Min shall follow the inject value of 4 mA, which is 3271?
b) The Input Max. value shall follow the inject value of 20 mA, which is 16368?

The field device is a 4 - 20 mA pressure transducer.

2) while for another analog input module;

Inject (mA) Value

4 3274
8 6551
12 9827
16 13106
20 16383

But at the SCP instruction

Input = I:7.1 (0)
Input Min. = N7:58 (1507)
Input Max. = N7:59 (0)
Scaled Min. = N7:68 (0)
Scaled Max = N7:69 (5500)
Output = N7:160 (5500)

c) The Input Min. is larger than Input Max.?

The field device is a pressure transducer using 4 - 20 mA.

Both analog input module is 1746-NI4.
 
Your assumptions are essentially correct. The previous programmer may have been getting creative:

1) He may have found the pressure transmitter in the field had some error or drift, and put in actual data and transmitter display readings to "calibrate" the signal.

2) The second transmiter may have been a suction side transmitter and he wanted to have the engineering units display negative.

There is, of course, also the possibility that he didn't know what he was doing.
 

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