294 • Common MODBUS functions
Watchdog (Fieldbus failure)
Modular I/O System
ETHERNET TCP/IP
6.2.12 Watchdog (Fieldbus failure)
The watchdog serves for monitoring the data transfer between the master con-
trols and the coupler/controller. For this the master cyclically actuates a time
function (Time-out) in the coupler/controller.
In the case of fault free communication, the watchdog timer does not reach its
end value. After each successful data transfer, the timer is reset.
If this time has elapsed a fieldbus failure has occurred.
In this case, the fieldbus coupler/controller answers all following MODBUS
TCP/IP requests with the exception code 0x0004 (Slave Device Failure).
In the coupler/controller special registers are present for the selection and
status inquiry of the watchdog by the master (Register addresses 0x1000 to
0x1008).
After switching on the supply voltage the watchdog is not yet activated. First
step is to set the time-out value (Register 0x1000). The watchdog can be acti-
vated by writing a function code in the mask register (0x1001), which is une-
qual 0. A second activation possibility is to write a value in the toggle register
(0x1003) deviating from 0.
Reading the minimum trigger time (Register 0x1004) reveals whether the
watchdog fault reaction was activated. If this time value is 0, a fieldbus failure
is assumed. The watchdog can be restarted in accordance with the previously
mentioned two possibilities or using the register 0x1007.
If the watchdog is started it can only be stopped by the user for safety reasons
via a certain path (register 0x1005 or 0x1008).
