Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

82 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
event of message collision, where it can occur. Because DNP3 supports peer–peer
communications, any station can act as a primary, or message initiator. Because of this,
messages can be sent from two stations simultaneously, causing a collision.
Before discussing the physical layer procedures, a brief review of the terminology is
given in the following table.
Simplex:
Half duplex:
Full duplex:
Two-wire
Four-wire
Communications are in one direction only
Two-way communication is possible, but only in one
direction at a time. This is because only one
communication path or channel is used. To effect
communication a protocol for handing over the channel is
required.
Simultaneous two-way communication is possible. Two channels are
provided to do this.
A two-wire connection will provide one communication channel. A
PSTN line is an example of this. Therefore, half-duplex
communications only are possible via this channel.
A four-wire connection will provide two channels, and is capable of
supporting duplex communications.
In the following procedures, the data carrier detect (DCD) indication is used to
determine if the communications medium is free. Generally a time delay must be used to
prioritize access to the channel once it has become free.
5.3.5 Half-duplex procedures
Direct link
With a direct, or point-to-point link, the delay from DCD clear needs to be just sufficient
to allow the master station to detect loss of DCD and begin transmission. Note that if a
dial-up connection is being used, the DCD signal indicates the establishment of a link
rather than the presence of data. In this case the RTS and CTS signals must be used. DNP3
must assert the RTS signal and await the CTS signal before transmitting each frame.
Multidrop link
In the case of a multidrop link, time delays must be used to ensure that stations get access
to the medium. This is accomplished by providing a back-off time made of a fixed delay
plus a variable delay. These delay times are configurable by the user. Normally the master
station will be given a zero fixed delay time, so that it can always gain access in a halfduplex
environment. Prioritization of slave stations can be accomplished by this method.
Again, the minimum delay must be long enough for the master station to be able to gain
control of the medium before any outstation.