Number of SCADA nodes required

This is probably the first thing to decide. How many SCADA nodes do you need?

The number of SCADA nodes you need depends on:

·         The Physical Location of your Automation Devices. Are they all close enough that they can connect to a single SCADA node?  If yes, then one SCADA node might be the answer.   

If the automation devices are far apart and cannot be connected to a single SCADA node, then you need a SCADA node for each location.

·         The automation network used.

o        Does it support Multiple Masters?  If not Multi Master, can the automation network be split into multiple smaller networks? If the answer is no to both, then one SCADA node is required.

·  Reliability. Multiple SCADA nodes provide a distributed architecture that is more reliable.  WebAccess also provides Redundant SCADA nodes that will automatically switch to the backup and redirect Clients to the backup SCADA Node.

A Hot Standby might be an inexpensive solution, where a second PC is loaded with SCADA node software and the user must move the license key to the hot standby PC. 

·         Trending. The number of tags that can be trended by a single SCADA Node are limited by:

o        The CPU speed,

o        The Disk Access time,

o  The SCAN time on the Com Port

o        The data log Deadband given to each tag and

o        The number of tags that exceed their data log deadband during each second. 

The upper limit is 800 tags requiring disk access changing more than their deadband in each second for a 1.8 GHz PC with a 7200 rpm IDE Hard drive. If you have larger requirements for Data Log trends, then you need to distribute your tags over multiple SCADA nodes or examine hard drive technology with large cache and faster write access than IDE (e.g. SCSI, SAN, etc.) For more information see 1.4.5 Disk Space and CPU speed