See what a healthy hot standby LED looks like for Schneider Electric M580 PLC. This video will help baseline a healthy state and make appropriate plans for debugging.
The M580 PLC comes with a hot standby diagnostic LED (HSBY Diag) which is located on the bottom of the HSBY Link port. When you connect only one port, the HSBY Diag LED will blink green once per seconds, and if both the links are connected and working correctly, on both the M580 PLC controller HSBY Diags LED will lite solid green.
Note that the HSBY(hot standby) can be either copper or fiber. In my demo, I am using a copper (Ethernet) cable. I am using a Finisar fclf8521P2BTL-SH transceiver.
In a nutshell, the HSBY Link LED should always be solid green even if the PLC programs are not equal. Any blinking denotes there is a problem.
If you are wondering, the HSBY SFP socket (Ethernet RJ45) part number from Schenider Electric is 490NAC0100. And it looks like the image shown below.
The pinout for the ethernet cable is as shown below.