Rockwell Automation 1797-OE8H FLEX Ex HART Analog Modules User Manual - Manual (Page 103)

Publication 1797-6.5.3 - March 2006
Apply FLEX Ex Analog I/O Modules 6-5
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:
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=440-273-10=157
Ω
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).
Rtotal =
(23.7-14)
0.022
= 440 Ω
Cable_length =
157
2*0.1
= 785 m