What are the Comms states for a Direct (Multidrop) Outstation with PSTN fallback capabilities?

Multidrop (MDRP) Healthy,PSTN Available.

In this state, the Master will send "polls" as configured.

For example, in DNP3:

  • Integrity,
  • Class 1 - 3,
  • Ethernet Keep Alive.

MDRP Failed, PSTN Available

In this state, the Master will attempt to re-establish multidrop communications at the interval specified in the Scan Parameters of the channel object (re-establishment interval). The "One Shot" and "Open Line" pick actions become available on the Outstation Object when in this state to allow users to force a PSTN connection to the outstation.

MDRP Failed, PSTN Healthy

This state occurs when either a "One Shot" or "Open Line" is performed. For DNP3, if a "One Shot" is actioned, then an integrity poll is sent to the OS - if any of the polling intervals are very short (approx < 3sec.), then a poll may be also sent.

If an "Open Line" is actioned, then the Master will send polls either whenever there is nothing else to do or, if the "Poll When Online" is off, as per the rates used for normal multidrop communications - the only difference being that communications will be disconnected after a given time (Maximum Call Length) OR if the user actions the "Close Line" pick action.


This State occurs when MDRP has failed AND either a "One Shot" or Open Line" has been unsuccessful a configured number of times. The Master will attempt to re-establish MDRP communications as per Scan Parameter Configuration.