Rockwell Automation 1794-APBDPV1 FLEX I/O PROFIBUS Adapter User Manual - Manual (Page 110)

Publication 1794-UM064B-EN-P - May 2014
98 Apply FLEX I/O Modules
P/I Analog Transmitter Functional and IS Parameters
The figure below shows a typical pressure to current analog transmitter. Two
types of characteristics are shown, functional and IS.
Loop Functionality Verification
The functional characteristics will be checked first. The figure below shows the
general situation which must be analyzed for proper loop function.
On the left are the module components of the voltage source to run the loop and
its internal source resistance. In the center is the cable. It represents a loss with
regard to its resistance. Cable resistance is a function of cable length. On the right
is the transmitter, which has a minimum voltage it must see to function properly.
The loop must function properly under maximum signal current conditions, in
this case 22 mA. As can be seen the key variable, which must be determined, is the
total loop resistance. This may be determined with the following equation:
600
250
14 16.4 24.7 42
4…20 mA IS
P/I transmitter
14…42V operate
EEx ia IIC T4
Ui=30V
Ii=165 mA
Ci=22.5 nF
Li=730 uH
Power Supply Voltage
External
load
resistance
41662
Ci<10 pF/m
Li<0.5 uH/m
R<0.1 Ω/m
Ca=118 nF
La=4 mH
Transmitter
minimum
voltage=14V
+
–
Cable resistance loss
–
+
Module source voltage=17V
Module source resistance=273 Ω
+
sig
10 Ω
Module sense
resistance=10 Ω
273 Ω
17V
Rtotal =
(23.7-14)
0.022
= 440
Ω