Rockwell Automation 1797-OE8 FLEX Ex Analog Modules User Manual User Manual
Page 71
Publication 1797-6.5.1 - May 2005
Applying FLEX Ex Analog I/O Modules 6-5
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”).
Rtotal =
(17-10.5)
0.022
= 295
Ω
Cable_length =
182
2*0.1
= 910m