On Demand Support in Advanced Modbus

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The current Advanced Modbus driver was developed with two main communication types: PSTN and direct. PSTN is designed to contact the outstation as and when needed to collect data. Direct support is designed around the idea that ClearSCADA has a permanent "always on" connection to the outstation and will poll the outstation at regular and frequent intervals.

If the connection between ClearSCADA and the outstation is via TCP/IP then the polling must be frequent enough to ensure the TCP/IP socket does not time out. Some deployments of ClearSCADA with SCADAPack Modbus or Generic Modbus outstations require a slow polling rate to either preserve power (if battery powered with solar panels) or to keep data exchanges to a minimum (e.g. GPRS connection).

The On Demand option has been added to all direct SCADAPack Modbus, Generic Modbus,RealFLO and Lufkin SAMS outstations. This gives users more flexibility when it comes to designing and configuring their ClearSCADA system.

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On-Demand support was added to the DNP3 driver in ClearSCADA 2009 R2.  This has been extended to the Advanced Modbus driver.  The feature works in exactly the same way as it does in the DNP3 driver.

 The On-Demand support replicates PSTN support but for TCP/IP connected devices in ClearSCADA.  To that end ClearSCADA will not automatically connect to the outstation.  But will require a user action, e.g. Control, Refresh to trigger the connection to the outstation.  If required the Schedule object can be used to trigger a regular periodic connection to the outstation.

On-Demand support can also be used in conjunction with the Comms.  Availability to allow access to the device at certain times only.  If Comms availability is also enable requests such as controls can be queued up and sent when we enter a period of available comms.

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