Port-level Redundancy

Normally, Plant SCADA port control involves opening a port at startup and closing it at shutdown (except when used in a scheduled device or dialup situation). However, if an RTU supports multiple IP connections, it will normally get a connection from the primary AND secondary port (normal Plant SCADA redundancy). This may not be acceptable for an RTU.

To help resolve this, ExclusiveAccess mode was introduced in to the DNPR driver (see Port-based Parameters).

Exclusive Port Access

When you connect to devices like terminal servers, which only support one connection at a time, port control is required.

An Citect.ini parameter has been provided to set a port's mode to "ExclusiveAccess". In this mode, when any unit on a port is usurped (moved from one state to another) from being active to inactive, the port connection is dropped. This will allow the port on the redundant server to have access to the device(s), bringing any associated units into action.

When using ExclusiveAccess mode, ports on both sides need to have the Citect.ini parameter set. Additionally, the virtual unit on the port needs to follow the active port, as the physical port may not be online (e.g. the RTU is offline).

Hardware which only allows one connection should have an "inactivity timer" set, so the connection is dropped if it is not used. For example, a terminal server needs to drop its connection if no communication has occurred after "x" seconds. This allows access to the unit from another server if the current connection is broken.

If the hardware doesn't support this, you may want to consider using the –K option on the TCP/IP driver combined with registry "Keep Alive" changes.