User manual

Publication 1797-6.5.1 - April 1999
6-4 Applying FLEX Ex Analog I/O Modules
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 22mA. As can be seen the key variable, which must be
determined, is the total loop resistance. This may be determined with the
following equation:
Once Rtotal is known, you must determine the actual cable resistance that
can be accommodated. This is the difference between Rtotal and the module
internal source and sense resistances: Rcable=295-91-22=182Ω
Rcable is used to determine the wiring length allowed between the module
and the transmitter:
Note, the “2” in the denominator of the equation is necessary to account for
the resistance of both cable conductors (signal source “+” and signal and
return “sig”).
Ci<10pF/m
Li<0.5uH/m
R<0.1Ω/m
41664
IIB
Ca=560nF
La=10mH
transmitter
minimum
voltage=10.5V
+
–
cable resistance loss
–
+
module source voltage=17V
module source resistance=91Ω
+
sig
22Ω
module sense
resistance=22Ω
91Ω
17V
Rtotal =
(17-10.5)
0.022
= 295Ω
Cable_length =
182
2*0.1
= 910m