Limitations of Enhanced Failover

General Limitations

When using Enhanced Failover with iFIX, be aware of the following general limitations:

  • If you did not purchase a key that supports Enhanced Failover, you will not be able to use this feature in iFIX. Do not use a standby SCADA Server as a development node. The standby SCADA node must be a dedicated standby node.
  • The SCADA pair must be configured identically.
  • You cannot make database modifications to the standby node. Data cannot be written to a standby node; you can only read data from the standby node. However, you can use Maintenance Mode on the primary node to temporarily suspend synchronization between the two SCADA nodes, so that you can make changes to your database while SAC is still running.
  • iFIX I/O drivers do not support redundant communication with multiple SCADA nodes. iFIX only supports synchronization of the Simulation (SIM) and Simulation 2 (SM2) I/O drivers. All SIM and SM2 driver information is synchronized between the active and standby nodes.
  • I/O drivers (6.x and 7.x versions) reading data between the SCADA pair is not supported. The I/O drivers on the active and standby SCADA nodes run independently of each other (for example, they have separate poll tables). Both SCADAs are reading data from the PLC/RTU network at all times. Therefore, the driver configurations running on both SCADA nodes should be identical.

NOTE: Be aware that if your driver is utilizing Access Time for its datablock/poll records, on the standby SCADA, these datablock/poll records will time-out.

NOTE: If both SCADAS are communicating to the same PLC, you can reduce the load by disabling a driver or I/O devices under a driver if a SCADA is in Standby mode. For more information, see:

  • Disable The IGS Driver on an iFIX Enhanced Failover Standby Scada
  • Enable or Disable Datablocks, Devices or Channels in a 7.x Driver Programmatically
  • The iFIX OPC data server and the Alarm & Event server running on the SCADA nodes is supported; however, the iFIX OPC servers do not support logical names. OPC clients will not automatically switch to the active node.
  • The Proficy Historian collectors should be running on both the active and standby iFIX SCADA nodes and configured in Proficy Historian as redundant collectors. For more information about working with Proficy Historian, refer to the Proficy Historian and Enhanced Failover section.
  • You should not have your Proficy Historian Archiver running on either computer in SCADA pair when using Enhanced Failover or LAN Redundancy.
  • If you upgraded from a version of iFIX before iFIX 5.0, and previously used the SCADA Failover feature, you will need to reconfigure some settings in the SCU (the data transport for synchronization) before Enhanced Failover will work. For more information, refer to the Enhanced Failover and Upgrading section in the iFIX Getting Started guide.
  • If you use Terminal Services with iFIX, be aware that a Terminal Server cannot reside on either the primary or secondary iFIX SCADA nodes configured for Enhanced Failover. If the SCADA Server and Terminal Server run on the same machine, that machine becomes a single source of failure.
  • The primary and secondary SCADA pair must be the same version (major and minor) of iFIX. For example, if the primary SCADA has iFIX 5.0 installed, the secondary SCADA must have iFIX 5.0 installed. The iClient nodes can be any version of iFIX.
  • You can have up to three network cards (a primary, secondary, and tertiary network path) configured for SCADA-to-SCADA communication.
  • If you are using UDP, the primary SCADA must have a dedicated network.
  • If the iClient cannot access the active node through iFIX networking, but can access the standby SCADA through iFIX networking, the iClient will use the data on the standby node. This will allow the Alarm summary to display the current value, but it won't get any new alarms until it can access the active node.
  • Dynamic Connections on the SCADA pair should be disabled for iClients to failover to partner SCADA in a timely fashion.
  • If you are using UDP, the primary and secondary SCADA computers must be physically next to each other in the same location/room.

Unsynchronized Items

Enhanced Failover only provides for database and alarm synchronization. The following items are not synchronized between the active and standby node with the Enhanced Failover feature enabled:

  • The background scheduler running on both SCADA nodes is not synchronized.
  • I/O drivers (other than SM2 and SIM) are not synchronized.
  • System Extension Toolkit (STK) modules are not synchronized.
  • The alarm printer, file, and history queues are not synchronized.
  • The Auto Alarm Manager (AAM) queue is not synchronized.
  • Any EDA background programs are not synchronized, unless the program is aware of the redundant SCADA setup.
  • iFIX Pictures are not synchronized.
  • Background schedules are not synchronized.
  • VBA scripting variables.
  • The AR (Analog Register) and DR (Digital Register) block values are not synchronized.

NOTE: The AR and DR blocks are not part of SAC processing, and therefore not part of the synchronization of the values between the two databases. The AR and DR blocks on the standby and active nodes reflect the value(s) transmitted by the local driver for the specified I/O address. Be aware that if the AR and DR blocks are not reading value from the same PLC, it is possible that the active and standby nodes to have different values for the AR and DR blocks in each database.