With some advanced drivers, the scan process differs slightly to that documented within this section. For example, some drivers can broadcast messages at the beginning of a System Scan. Where scanning by a particular driver differs, any differences are explained in the driver-specific documentation.
Systems that use independent connections can scan all the outstations on a direct network independently of each other, see Independent Connections. With direct a permanent connection is maintained between the ClearSCADA server and each outstation.
The actual scanning process varies, depending on the protocol. With direct outstations on many advanced drivers, scanning is typically achieved using two scan phases. With many advanced drivers, the driver typically scans each of the outstations on a channel for current data (the Primary Phase of the scan) and then scans the outstations for secondary (logged) data (the Secondary Phase of the scan).
The complete process (retrieval of both current and logged data) is referred to as a System Scan.
The exception to this process is when the ClearSCADA server has not been in, or has regained, communications with an outstation. In such circumstances, the driver typically performs an Establishment Phase scan in place of a Primary Phase scan (see Establishment Phase).
Normally, system events will primarily be detected during current data extraction, with retrieval of event logs and timed logs (buffers) having secondary importance.
This example illustrates how outstations are scanned on a system with independent connections.
A driver scans four outstations that are using a direct channel with independent connections to communicate with the server. The time required to scan each outstation is independent of all the other outstations that are being scanned at the same time.
For each driver that uses direct communications on your system, you need to specify the frequency with which the driver is to scan each outstation for current data and / or logged data. Depending on the protocol, you specify the required data retrieval or scan intervals on either the Channel Form or the Outstation Form (see the driver-specific documentation for details).