In a Citect SCADA system, a computer can act as a server, or a client, or both. Servers are used to manage communication with plant equipment and collate production data, while clients provide the interface to assess and interact with the system. A topology allows you to view how your servers and clients are arranged.

There are several different types of server processes used in a Citect SCADA system. These include:

• Alarms server process
• I/O server process
• Trends server process
• Reports servers process.

How you arrange these sever process types across the computers included in your system topology will be determined by the following:

• Operational requirements — a standalone system may contain a single computer that hosts every component of a system, whereas a distributed system may have a separate computer dedicated to each server type.
• Redundancy — the servers associated with a system can be duplicated and defined as primary and standby units, allowing the system to keep running if one of the servers becomes inoperative.
• Clusters — Clustering allows you to group independent sets of Citect SCADA's server components within a single project, allowing multiple systems to be monitored and controlled simultaneously. The clusters offer the benefit of keeping a logical structure to the project during configuration.

Citect SCADA's configuration environment allows to you view a system topology as a computer-based view or a cluster-based view.

I/O devices can include any control or monitoring equipment with a communication port or data exchange interface, such as programmable logic controllers, loop controllers, bar code readers, scientific analyzers, remote terminal units, or distributed control systems.

To incorporate an I/O device into a system topology, you will need to install the relevant driver. A driver allows an I/O device to communicate with a Citect SCADA I/O server.

You also need to configure the various components required to enable runtime communication with the device. This can include boards, ports and modems. You will need to consider the following:

• Transport medium — the physical communications medium and the low-level logic necessary to drive it.
• Protocol — typically a device will support an industry-standard communications protocols, such as Modbus, OPC, DNP 3.0 or BACnet. This will, however, depend on the device.

Much of the functionality supported by the protocols and transports you select can be modified using the driver options and parameters. Each driver has its own specific section in the online help that will guide you through configuration options.

To help you get started, Citect SCADA includes a Device Communications Wizard that can assist with the configuration of the communications settings for a particular device.