Question about 4 wire smokes and ELK

[RogueOne]

I have 2 4wire smokes I want to connect to my M1gold.  I have one in the basement and one upstairs.  I understand I need to use a EOL resistor on the last smoke but I read that its also recommended to use a eol relay and I don't even know what that is.  Is this required and what does it do?
 
Thanks

 

[mikefamig]

There is a wiring diagram on page 6 of the m1 installation manual. The relay is to supervise the power to the smoke alarm. If there is an interruption to the power the relay will open causing the zone to open alerting the m1 to the problem.
 
Mike.