Kidde ARIES Installation, Operation And Maintenance Manual page 57

Note: Isolator modules are not required for a Class-B, Style-4 signaling line
circuit. A maximum of 30 SmartOne devices can be installed between
a pair of isolator modules.
Figure 2-12. Class-B, Style-4 Signaling Line Circuit
P/N 06-236530-001 2-15 August 2013