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Troubleshooting

9-24

SIMATIC 545/555/575 System Manual

9.10 PROFIBUS-DP Communications Watchdog Timer

Most slaves have a communications watchdog timer that may be activated

or deactivated by the user during initial configuration. The communications

watchdog timer is distinct from a hardware watchdog timer. The

communications watchdog timer monitors time between communications

from the PROFIBUS-DP annex card. If the communications watchdog timer

is activated and communication between the PROFIBUS-DP annex card

and the slave is broken, the communications watchdog timer on the slave

will expire and set the output points to a safe state. The safe state may vary

depending on the slave’s design, but most slaves “zero” the output points.

Refer to your slave manual(s) for more information about slave watchdog

timer reaction.

The PROFIBUS-DP annex card restarts communications to the slaves in

the following events:

• If an error has been detected by the CPU, when the error is cleared by

the CPU.

• When communication between the PROFIBUS-DP annex card and the

CPU is restored.

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