know its device address, recognize a message addressed to it, determine the kind of
action to be taken, and extract any data or other information contained in the message.
If a reply is required, the controller will construct the reply message and send it using
Modbus protocol.
This conversion also extends to resolving node addresses, routing paths, and error–
checking methods specific to each kind of network. For example, Modbus device
addresses contained in the Modbus protocol will be converted into node addresses prior
to transmission of the messages. Error–checking fields will also be applied to message
packets, consistent with each network’s protocol. At the final point of delivery, however
– for example, a controller – the contents of the imbedded message, written using
Modbus protocol, define the action to be taken.
DNP3
DNP3 is a protocol for transmission of data from point A to point B using serial
communications. It has been used primarily by utilities like the electric companies, but it
operates suitably in other areas. DNP3 is a non-proprietary protocol that is available
worldwide with no restrictions.
The DNP3 protocol allows a server to send binary input data that is useful to monitor
two-state devices. For example a circuit breaker that is closed/tripped or a pipeline
pressure alarm that shows normal or excessive pressure would both be types of binary
data sent by the server. It also allows a server to send analog input data that conveys
information like voltages, currents, power, reservoir water levels and temperatures. In
addition, it allows the sending of count input data and finally, it allows the sending of
files that contain configuration data.
The DNP3 protocol allows a client to issue control commands. For example, to close or
trip a circuit breaker, raise or lower a gate, and open or close a valve. It could also allow
the sending of analog output values to set a regulated pressure or set a desired voltage
level. It also would allow computers to talk to each other about synchronizing the time
and date, sending historical or logged data and waveform data.
DNP3 was designed to optimize the transmission of data acquisition information and
control commands from one computer to another. It is not a general purpose protocol
for transmitting hypertext, multimedia or huge files. The terms server and client are
applicable to DNP3 systems. A server is a device or software process that has data or
information that someone else wants. A client is a device or software process that
requests data from a server.
DF1
DF1 protocol is a proprietary data-link layer protocol that combines features of
subcategories D1 (data transparency) and F1 (two-way simultaneous transmission with
embedded responses) of ANSI x3.28 specification. There are two categories of DF1
protocol, half-duplex protocol and full-duplex protocol.
A link protocol is a set of programming rules for interpreting the signals transmitted over
a physical link. The DF1 protocol carries a message, error free, from one end of the link
to the other. It has no concern for the content of the message, the function of the
message, or the ultimate purpose of the message. For example, with full-duplex DF1
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