Watchdog

When using pysmlib all the FSM logic is not directly connected to the EPICS IOC as is the case for the EPICS sequencer. This means that if the pysmlib executable crashes or it loses network connection, all the FSM logic will stop to work, while the IOC continues to live without noticing it. In some cases this can be a problem and you may want at least to trigger a warning for someone to check the situation. For this reason a mechanism has to be implemented to inform the IOC about the “online” status of the FSM executable.

An easy way of doing it is to implement a watchdog logic, that is define a special input where to perform a periodic put() and signal an “offline” status when no put() is received for a time longer than the period.

IOC side: definition of the PV

For this purpose a special kind of PV can be used: a binary output. This particular record type has a field called HIGH which sets the time its value must remain high (that is to 1) after receiving a put(1). So, it is sufficient to write to it from a FSM with a smaller period to keep it always at 1. Then, if the value goes to 0 the FSM is recognized as offline.

The PV (one for each FSM) can be defined like this:

#watchdog
record (bo, "watchdog") {
    field (DESC, "FSM watchdog")
    field (DTYP, "Soft Channel")
    field (DOL, 0)
    field (HIGH, 20)  # keep the 1 value for 20s after the put
    field (PINI, 1)
    field (ZNAM, "Disconnected")
    field (ONAM, "Connected")
}

Refer to the EPICS documentation for more informations on how to define PVs inside an IOC.

FSM side: the watchdog input

To signal being online each FSM has to perform periodic put() to its watchdog PV. This can be easily achieved with the setWatchdogInput() method of fsmBase: it is sufficient to pass to it a standard input (created with connect()) and set two parameters:

1. The watchdog mode, which can be:

   1. “on-off”: A put is performed periodically, once to 1 and once to 0.

   2. “off”: A put(0) is performed periodically.

   3. “on”: A put(1) is performed periodically.

2. The watchdog period in seconds.

Example

In the following example the input wdog is used as watchdog. A put(1) will be automatically performed to it every 5s, as long as the FSM is running.

class exampleFsm(fsmBase):
    def __init__(self, name, *args, **kwargs):
        super(exampleFsm, self).__init__(name, **kwargs)

        self.wdog = self.connect("exampleWdog")
        self.setWatchdogInput(self.wdog, mode="on", interval=5)

    ...