I studied the technologies about how to transmit data and power on two-wire and found an answer for this issue.
The result of study show that analog 4–20 mA current loops are commonly used for electronic signalling, with the two values of 4 & 20 mA representing 0–100% of the range of measurement or control. These loops are used both for carrying sensor information from field instrumentation, and carrying control signals to the process modulating devices, such as a valve
In the case of a fire alarm, 6 mA could be normal, 15 mA could mean a fire has been detected, and 0 mA would produce a trouble indication, telling the monitoring site the alarm circuit had failed.
Some 4–20 mA field devices can be powered by the current loop itself, removing the need for separate power supplies, and the "smart" HART protocol uses the loop for communications between field devices and controllers. Here are some links:
However, I haven't make sure that the addressable fire alarm Systems are using.
Can anyone help me confirm that this is right or wrong?