Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Fundamentals of distributed network protocol 81
As previously noted, the physical layer defines the physical interface characteristics in
terms of electrical specifications, timing, pin-outs and so on. This includes details required
to establish and maintain the physical link. The data element at this level is essentially the
bit, ie it is concerned with how to pass one bit of data at a time.
5.3.1 Description of physical layer
The physical layer originally recommended for DNP3 has the following specification:
• Bit serial asynchronous
• 8 data bits
• 1 start bit, 1 stop bit
• No parity
• RS-232C voltage levels and control signals
• CCIT V.24 hardware protocol for DTE/DCE communications
The DNP3 Users Group Technical Committee subsequently produced a standard for
transmission of DNP3 over networks. This provides an alternative definition of the physical
layer for that situation.
5.3.2 Services provided by physical layer
The physical layer must provide these functions:
5.3.3 Topologies
• Connect
• Disconnect
• Send
• Receive
• Status
As previously described, DNP3 supports either master-slave or peer-to-peer communications,
with either one-on-one or multidrop topologies.
• Point-to-point
• Multidrop from one master
• Hierarchical with intermediate data concentrators
• Multiple master
Point-to-point, or direct topology refers to the case of two DNP3 devices connected
together, either directly with a cable, or via modems and a communications path. This
could be via a leased line, via radios, or via the public switched telephone network
(PSTN).
A serial bus topology is the alternative to a direct topology. This may also be referred to
as multidrop. In this case multiple devices are connected to the same communications
path.
5.3.4 Physical layer procedures
Procedures must be provided to support both half-duplex and full-duplex communications.
The specific procedures used will depend on the topology as well as whether half- or fullduplex
transmission is available. One particular role of the procedures is to manage the