Behavior Differences Between Standard And Fail-Safe Cpus; Safety Mode; Virtual Monitoring Number - Siemens SIMATIC S7 Functional Safety Manual

You can exchange data between the safety program and the standard user program in the
fail-safe CPU by means of bit memory or data of a standard DB.
1.3.1

Behavior differences between standard and fail-safe CPUs

1.3.1.1

Safety mode

Safety mode
In safety mode, the safety functions for fault detection and fault reaction are activated in the
following:
● Safety program of the fail-safe CPU
● Fail-safe signal modules (SM)
Safety mode of safety program
The safety program runs in the fail-safe CPU in safety mode. The safety program activates
all safety mechanisms for fault detection and fault reaction. You cannot modify the safety
program during operation in safety mode.
You can deactivate and reactivate the safety mode of the safety program. "Deactivated
safety mode" enables the safety program for online tests and changes as needed while the
fail-safe CPU is in RUN mode.
For SIMATIC Safety, you can switch back to safety mode only after an operating mode
change of the fail-safe CPU from RUN to STOP to RUN.
Safety message frame
In safety mode, the fail-safe CPU and fail-safe SMs consistently transmit data between them
in a safety message frame. The safety message frame in accordance with PROFIsafe
standards consists of the following:
● Process data (user data)
● Status byte/control byte (coordination data for safety mode)
● Virtual Monitoring Number (encoded in CRC signature, provides keep-alive mechanism
and detection of out-of-sequence messages)
● CRC signature
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
Product overview
1.3 S7-1200 Fail-Safe CPUs
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