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MODBUS Functions • 177Internal VariablesWAGO-I/O-SYSTEM 750ETHERNET TCP/IP6.5.1 Description of the internal variables6.5.1.1 Watchdog (Fieldbus failure)The watchdog monitors the data transfer between the fieldbus master and thecontroller. Every time the controller receives a specific request (as define inthe watchdog setup registers) from the master, the watchdog timer in thecontroller resets.In the case of fault free communication, the watchdog timer does not reach itsend value. After each successful data transfer, the timer is reset.If the watchdog times out, a fieldbus failure has occurred. In this case, thefieldbus controller answers all following MODBUS TCP/IP requests with theexception code 0x0004 (Slave Device Failure).In the controller special registers are use to setup the watchdog by the master(Register addresses 0x1000 to 0x1008).By default, the watchdog is not enabled when you turn the controller on. Toactivate it, the first step is to set/verify the desired time-out value of theWatchdog Time register (0x1000). Second, the function code mask must bespecified in the mask register (0x1001), which defines the function code(s)that will reset the timer. Finally, the Watchdog-Trigger register (0x1003)must be changed to a non-zero value to start the timer.Reading the Minimum Trigger time (Register 0x1004) reveals whether awatchdog fault occurred. If this time value is 0, a fieldbus failure is assumed.The timer of watchdog can manually be reset, if it is not timed out, by writinga value of 0x1 to the Restart Watchdog register (0x1007).After the watchdog is started, it can be stopped by the user via the WatchdogStop register (0x1005) or the Simply Stop Watchdog register (0x1008)6.5.1.2 Watchdog Register:The watchdog registers can be addressed in the same way as described withthe MODBUS read and write function codes. Specify the respective registeraddress in place of the reference number.Register address 0x1000 (MODBUS Address 404097)Designation Watchdog time, WS_TIMEAccess read / writeDefault 0x0000Description This register stores the watchdog timeout value as an unsigned 16 bit value.The default value is 0. Setting this value will not trigger the watchdog.However, a non zero value must be stored in this register before the watchdogcan be triggered. The time value is stored in multiples of 100ms (e.g., 0x0009is .9 seconds) It is not possible to modify this value while the watchdog isrunning. |
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