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How is data transferred in a SCADA systems?
Data transfer between the PLCs and the HMI can be accomplished using many different communication technologies. Currently available technologies include:
- Hard-wired: this connection type utilizes the RS-232 and RS-485 communication standards and takes advantage of wiring networks found in most industrial plant installations. Each Control Microsystems PLC is equipped with a number of RS-232 and RS-485 communication ports.
- Dial-up: this connection type uses the existing telephone line infrastructure to transfer data. In addition to a PLC RS-232 port, an external dial-up modem is required at both the HMI and the PLC ends. Control Microsystems’ 5901 Dial-Up modem is ideal for this type of communication.
- Wireless (radio): this communication type utilizes a number of radio technologies, including UHF and Spread-Spectrum. In all cases, a radio is required at both the HMI and the PLC ends.
- UHF radios use frequencies in the range 300MHz and 3 GHz. This band of frequencies allows for compact receiving antennas and high signal attenuation. UHF radio communication requires a PLC RS-232 port, a Control Microsystems 5902
Bell 202 modem and a UHF radio. The 5902 modem converts the digital bit stream from the PLC to audio tones, for transmission by the UHF radio.
- Spread spectrum radios use transmission signals in a bandwidth considerably greater than the frequency content of the original information. This type of communication uses techniques such as frequency hopping to decrease the potential interference to/from other receivers/transmitters. Spread spectrum radio communication typically requires an RS-232 port on the PLC and a spread spectrum radio. Control Microsystems’ Wireless series of PLCs, has an integrated spread spectrum radio in either FreeWave or MDS radio formats. The Control Microsystems’ 5908 and 5908-W02 modules provide radio-only functionality, again in both FreeWave and MDS radio formats, for the PC running the HMI software and other devices that do not have integrated radios.
- OPC Server: OPC is OLE for Process Control. It is a communication technology that provides interoperability between products from many vendors. The standard OPC interface allows makers of Human-Machine-Interface software, custom application developers and makers of field hardware, including PLCs, to know that they will be able to communicate with others. It also allows multiple applications and hosts to share communication networks with field devices. SCADAServer is Control Microsystems’ proprietary OPC server. It provides a common interface between third-party OPC clients, Control Microsystems applications and field devices such as the SCADAPack series of controllers based on the OPC automation control standard. SCADAServer supports the following connection types:
- Serial port connection (hard-wired, radio and dial-up)
- Internet Client connection
- Modbus Server connection
- Internet: This type of connection utilizes corporate LANs (Local Area Network) and WANs (Wide Area Network) to transfer data at higher rates than serial communication. External hardware, such as cables, switches, hubs and LAN/WAN infrastructure are required to interface the PC Ethernet port to the communication network.
Each Control Microsystems’ PLC serial port is independently configured and operates independently of the others. This means that the afore-mentioned communication technologies can be used together in the same SCADA System and that the PC and PLCs can be located in the same industrial plant or on opposite sides of the world.
