UT_WATCHDOG — Comms / heartbeat monitor and generator

Tells you a remote device, link or task is alive: it watches a value that should
keep changing (a heartbeat bit or counter from a remote PLC, a Modbus register, an
MQTT message counter) and raises CommsFail when it stops. It also produces a
heartbeat for the other side to watch.

Pins

	Pin	Type	Default	What to wire
in	`Beat`	BOOL	FALSE	Heartbeat bit from the remote (any change counts)
in	`Counter`	DINT	0	Or a counter/register from the remote (any change counts)
in	`Enable`	BOOL	TRUE	FALSE: not monitored (`Ok` and `CommsFail` off); on again starts over
in	`Reset`	BOOL	FALSE	Rising edge: reset a held failure (panel button)
out	`Ok`	BOOL		Remote alive (off at start until the first change)
out	`CommsFail`	BOOL		No change for `timeout`
out	`HeartbeatOut`	BOOL		Toggles every `beatPeriod`; send this to the remote
out	`HeartbeatCount`	DINT		Increments every `beatPeriod`
inout	`Data`	UT_WATCHDOG_DATA

Record `UT_WATCHDOG_DATA`

Field	Type	Default	Meaning
`timeout`	TIME	T#10s	No change for this long = link dead. Make it longer than the remote's beat and the PLC scan (T#0s: always failed)
`recoverTime`	TIME	T#0s	Must be alive this long before `Ok` returns (and AUTO clears `CommsFail`)
`beatPeriod`	TIME	T#1s	`HeartbeatOut` toggles every `beatPeriod` (0 = off)
`failReset`	UT_RESET_MODE	UT_RST_AUTO	AUTO clears by itself; RETRY as AUTO three times an hour, then as MANUAL; MANUAL holds `CommsFail` until reset
`cmdReset`	BOOL		HMI: reset a held failure
`cmdResetCount`	BOOL		HMI: `failures` and `downTime` back to 0
`ok`	BOOL		Status: same as `Ok`
`lastChange`	TIME		Status: time since the remote last changed
`failures`	UDINT		Status: comms failures
`downTime`	TIME		Status: total time dead, from each timeout to the next change

How it works

The failure is a UT_PROTECT trip, so the reset modes work as on every
device. A reset while the link is still dead is refused.
A link that drops out again while recovering fails again (and is counted again).
A link that is dead at start counts as a failure once timeout has passed.
The heartbeat keeps going when Enable is off. A beatPeriod shorter than the scan
toggles every scan.
A held command acts once.
Power cut: nothing is kept. failures and downTime start at 0, a held (MANUAL)
failure is forgotten, and a link that is still dead fails again after timeout.

Using it

LinkWd(Counter := RemoteHeartbeatReg, Data := LinkWdCfg);
RemoteHeartbeatOut := LinkWd.HeartbeatCount;     (* write to the remote *)
CommsAlarm(In := LinkWd.CommsFail, Data := Alarms[5]);

OpenPLC

UT_WATCHDOG — Comms / heartbeat monitor and generator

Pins

Record UT_WATCHDOG_DATA

How it works

Using it

Record `UT_WATCHDOG_DATA`