Johnson Controls AIM-200 Technical Manual page 18

Redundant Circuit
Figure 10: Redundant Circuit
Total length of the communications loop pair cannot exceed 10,000 feet
(measured from the control panel out, and back to the control panel).
The DC resistance of the communications loop pair cannot exceed
40 ohms. This measurement may be modeled by disconnecting loop out
and return at the control panel, shorting the two leads of communications
loop return together and metering loop out.
18 Addressable Intelligent Module—AIM -200 Addressable Intelligent Module