# PLC Watch-Dogging of RTU

The Catalyst RTU has its own internal watch-dogging intended to provide the strongest possible guarantee of mission readiness.

It is possible to add an additional level of system self-checking by using a PLC to monitor the health of the RTU. The strategy involves using the RTU DNA capability to provide the PLC with a "heartbeat,"

an indication that the RTU scan is active. When this heartbeat is not seen for a specified period of time, the PLC knows that the RTU is not scanning the PLC.

An example of ladder logic implementation of this strategy is described below. Please note that this strategy only provides an indication that the RTU scan is active. It will not detect failures involving other RTU subsystems, such as a severed speaker wire. Furthermore, indication failure does not necessarily imply that the RTU itself has failed (the RTU scan could be turned off, the PLC cable disconnected, etc).

Required PLC configuration: Uses 1 timer, 1 output relay, 1 discrete and 1 16-bit register.

EXAMPLE | SYMBOL | DESCRIPTION
--- | --- | ---
00010 | RTU_HEART | When set to '1' by RTU, failure timer is reset
40001 | RTU_ACCUM | Accumulator for RTU failure timer
#0400 | RTU_PRESET | 400 second trip delay for RTU failure timer
00001 | RTU_FAILED | Output relay for RTU failure indication

Required RTU Configuration: Uses 1 physical input and 1 channel.

Select one of the RTU physical inputs. Call it RTU1. Leave this input ungrounded so it will always have the logical value 1.

Configure any channel of the RTU as follows:

- SNA = 0*1*RTU1
- DNA = 1*1*RTU_HEART (assuming PLC is node 1)

|||||||||||||||||| RTU Watchdog Ladder Logic Diagram |||||||||||||||||||

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| RTU failure timer, always enabled, reset when RTU_HEART is 1

1+-------+----+--- - - - - - - - - - - - - - - - - - - - - - - - - --( )

| #0400 | 00001

| | |

2+--+/+--+T1.0+-

|00010 |40001|

| | |

| +-------+

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| Clear RTU_HEART every ladder logic scan

3+ ----- - - - - - - - - - - - - - - - - - - - - - - - - - - - ------( )

| 00010

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