I want to monitor all slaves independently

Hello:

could I ask you ?
I Have Multi Slave (CPU 313C-2DP) , I want to monitor these slaves by OB1 that contain SFC51.

my question is :
when any slave goes out the service , I want to monitor all slaves independently
e.g:
let's suppouse that slave1 in stop mode;
and something is wrong like module 2 within slave1. how can I discover that the slave 1 has a problem independently form another slaves?

Well you could code a watchdog that bounches a signal between the slaves and the master and when a signal timeouts you know that something is wrong with that slave.