Chapter 5 - Connection to a controller
Version EN-092019-1.21
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5.1.6
Connection monitoring
The Modbus register 9998 can be used to monitor the connection between the PLC and the gateway. The
value written to the register represents a timeout in milliseconds. Writing a value above 0 to the register
means that the connection status is monitored with the set millisecond time frame. If the gateway notices
that the register is not read or rewritten within the time frame defined by the value the error state
“watchdog expired” is set in the gateway.
In redundant systems the error state is also set in the redundant gateway (see
).
Note
Once the watchdog is activated it can not be deactivated by writing value 0.
In non-redundant systems with only one gateway, setting a watchdog timer entails that the PROFIBUS
master is set to STOP mode when the watchdog expires. The STOP mode corresponds to a fail-safe mode.
In redundant systems running with two gateways the PROFIBUS master of the different segments is not
automatically set to operation mode STOP. Instead, the error state must first be detected by the
redundant gateway. The PLC must track this register and must take appropriate action when the error
state “watchdog expired” is detected .
Register
Register Type
Meaning
9998
Modbus watchdog
register
This register represents a timeout in milliseconds (ms) and is used
to monitor connections between the PLC and the gateway.
5.2
Implementing Modbus with Unity Pro
Video
Watch the video
Modbus configuration with Unity Pro
for details on how to configure the
Modbus Gateway in Unity Pro to access the process data of the PROFIBUS devices via Modbus.
5.3
Implementing Modbus with TIA Portal
Video
Watch the video
Modbus configuration with TIA Portal
for details on how to address the Modbus
Gateway in TIA Portal to the process data of the PROFIBUS devices via Modbus.
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