Applies To:
  • CitectSCADA 5.50, 6.0, 6.1, 7.0
  • CitectHMI 5.50, 6.0, 6.1, 7.0
  • CitectFacilities 5.50, 6.0, 6.1, 7.0
  • CitectSCADA Batch 5.50, 6.0, 6.1, 7.0
  • VijeoCitect 5.50, 6.0, 6.1, 7.0


Consider a scenario where Citect SCADA Server is connected via TCP/IP to a number of devices of which some are inoperative (or not contactable), and the number of inoperative devices is sufficiently high (as of Windows XP Service Pack 2, this number is ten). If an operational device becomes inoperative, the communication between Citect SCADA and this device is unlikely to be re-established when the device becomes operational again.

During normal operation, applications may use TCP/IP to connect to any number of hosts. The number of connections is not limited. However, in Windows XP Service Pack 2, the number of “half-open” connections (those that are attempting a connection but have not yet completed, either by success or failure) is limited to ten[1], and any further connection attempts must wait for one or more of these “half-open” connections to complete before a new connection attempt can begin network activity.

During normal operation of Citect, many TCP/IP connections can be required and these will then be subject to the “half-open” limit that is in place. This will manifest as a longer initialisation time while the connections are being established, it will also cause Citect to consider a connection attempt to have timed-out before that connection attempt has even begun network activity.



As of the time of writing, the vendor of the operating system (Microsoft) has not supplied a work-around for this limitation.

In order to overcome this, a tool that modifies the operating system’s TCP/IP driver can be used to increase the “half-open” connection limit[2]. The user must be aware that this action is undertaken at their own risk, as such, the user should carefully consider the available information before pursuing this action.

The ABMLXEIP Driver has been used to demonstrate this re-connection failure scenario with as few as 15 devices. When the TCP/IP driver was modified to allow 50 “half-open” connections, this problem did not occur any more.