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why can't I control my safety modules (red flickering ERR led on the I/O modules) when I start my application

 Most likely, the safe communication is invalid. (see attached example screenshot)

- either the CPU is configured in NTP client mode, and NTP server is not present => in order to manage this situation,
-> use the safety device-DDT diag bits in the logic to warn if the NTP service is down :device DDT /S_COM_DBG/M_NTP_SYNC  and /S_COM_DBG/CPU_NTP_SYNC

- or, the CPU is configured in NTP server mode, but the safe communication is invalid => in order to revalidate the safety communication :
-> updated the real time clock of the CPU
-> set the process system word %SW128 to 16#1AE5 and then to 16#E51A .

important Note : when using a CPU that wasn't powered for a long time, prior to install it, it must anyway be powered, and the Real Time Clock must be set to present date & time using Unity Pro

Basics :
in order to deal with safety I/Os, a safety CPU and his remote I/O drop head (CRA) use their Real Time Clocks (RTC) to time stamp their safety exchanges, cross-checking the time stamps of his counterpart exchanges vs its own RTC in order to maintain the accuracy of their RTC, the CPU and CRA rely on the NTP protocol, and need a reachable NTP server.
if the NTP server is not reachable, then the RTC of each element (CPU and CRA(s)) are not updated, the safety communication synchro remains invalid, the safety communication remains invalid, and the I/O remain in fall back state.
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