There are 2 ways to control the status of FD40:

1. Detecting the "running" signal of asterisk

FD40 is controlled through an Asterisk loadable module res_failover.so. This module keeps sending "running" signal to FD40 when Asterisk is running. Once Asterisk is down or system power is cut off, the module will not be able to send out the "running" signal to FD40 and thus FD40 will automatically switch to the standby server.

2. Detecting the "running" signal of a independent daemon driver "failover"

The way how it works is similar with above, but it is controlled through a independent process and has nothing to do with asterisk or others. You can modify the diver to let the failover which you want it to be! 

More information please refer : http://downloads.openvox.cn/pub/drivers/failover/failover_daemon-1.0.tar.gz 



Important Notice:

The res_failover.so module is suitable for all versions of asterisk, including the latest 10.1.3, so you have nothing to worry about the compatibility. 

And another thing you need to know is that the compiled module "res_failover.so" is not always available for specific version of asterisk, so the best way to make it work fine is to compile it on your own.

Here is a manual to show you how to do:
http://downloads.openvox.cn/pub/manuals/old/English/FB40_FD40_User_Manual.pdf