Enabling Outputs In Stop Mode - Siemens S7-1200 System Manual

Online and diagnostic tools
14.11 Monitoring and modifying values in the CPU
For modifying a PLC tag at a given trigger, select either the start or the end of cycle.
● Modifying an output: The best trigger event for modifying an output is at the end of the
scan cycle, immediately before the CPU writes the outputs.
Monitor the value of the outputs at the beginning of the scan cycle to determine what
value is written to the physical outputs. Also, monitor the outputs before the CPU writes
the values to the physical outputs in order to check program logic and to compare to the
actual I/O behavior.
● Modifying an input: The best trigger event for modifying an input is at the start of the
cycle, immediately after the CPU reads the inputs and before the user program uses the
input values.
If you suspect values are changing during the scan, you might want to monitor the value
of the inputs at the end of the scan cycle to ensure that the value of the input at the end
the scan cycle has not changed from the start of the scan cycle. If there is a difference in
the values, your user program might be erroneously writing to inputs.
To diagnose why the CPU might have gone to STOP, use the "Transition to STOP" trigger to
capture the last process values.
14.11.4.2

Enabling outputs in STOP mode

The watch table allows you to write to the outputs when the CPU is in STOP mode. This
functionality allows you to check the wiring of the outputs and verify that the wire connected
to an output pin initiates a high or low signal to the terminal of the process device to which it
is connected.
Risks in writing to physical outputs in STOP mode
Even though the CPU is in STOP mode, enabling a physical output can activate the
process point to which it is connected, possibly resulting in unexpected equipment
operation. Unexpected equipment operation can cause death or severe personal injury.
Before writing to an output from the watch table, ensure that changing the physical output
cannot cause unexpected equipment operation. Always observe safety precautions for your
process equipment.
You can change the state of the outputs in STOP mode when the outputs are enabled. If the
outputs are disabled, you cannot modify the outputs in STOP mode. To enable the
modification in STOP mode of the outputs from the watch table, follow these steps:
1. Select the "Expanded mode" menu command from the "Online" menu.
2. Select the "Enable peripheral outputs" option of the "Modify" command of the "Online"
menu, or from the context menu after right-clicking the row of the Watch table.
You cannot enable outputs in STOP mode if you have configured distributed I/O. An error is
returned when you try to do this.
Setting the CPU to RUN mode disables "Enable peripheral outputs" option.
If any inputs or outputs are forced, the CPU is not allowed to enable outputs while in STOP
mode. The force function must first be cancelled.
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WARNING
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG