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.
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