The IEEE defines communications protocol as: a formal set of conventions governing the format and relative timing of message exchange between two communications terminals. A strict procedure required to initiate and maintain communication. This regulates the order and arrangement of
Information, transfer speed or baud rate and error checking.

In general, power system communication networks support four basic operations: establish communications terminate communications, write data, and read data. The write data function can be used to tell an IED to perform a control action, change settings, or send data to the requesting device.

Error checking is done by each device to determine if the message data was corrupted during transmission. The type of protocol, message format, and transfer speed are parameters that are configured during installation.

Communications schemes are polled, scheduled or unsolicited. In a polled situation, one IED acts as the host and initiates almost all data exchange. The other IED acts as a slave and does only what it is told.

The slave rarely initiates data exchange; it simply reacts to requests for data from the host. The exception is an unsolicited message from a slave which sends data to the host without the host requesting it. Often, this is a result of an unexpected change.

Popular Protocols
ASCII - Protocol that is easily converted to human-readable characters and numbers. This protocol is simple but generally slow.

Modbus® - A popular protocol with industrial users that has also become somewhat popular in substations. Designed to emulate PLCs transferring register data to one another.

Modbus® Plus - A medium speed network built with proprietary network interfaces using an extension of Modbus protocol.

DNP 3.0 - An ever increasingly popular SCADA protocol, governed by a standards committee and users group, that was designed to optimize efficiency through report by exception, remote modem connections, and multidrop capabilities. Predominantly popular in North America.

UCA/MMS - Utility Communications Architecture, currently being designed by North American utilities, vendors, and consultants to satisfy most requirements in substation feeder equipment and eventually all power system equipment.

Proprietary - Protocols created by the product vendors to communicate with their devices. These are generally unique for each vendor and are not inter-operable. Some vendors design their own protocol because existing protocols lack necessary robustness and efficiency.

Interleaved - Interleaved data streams is a simple way that multiple communications messages can occur on a single communications connection. Data acquisition, control, configuration, and time synchronization communications can occur at the same time.

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