22. Serial Protocols
ROX™ v2.2 User Guide
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RuggedBackbone™ RX1500
22.1.6. DNP (Distributed Network Protocol)
The RX1500 supports DNP 3.0, commonly used by utilities in process automation systems. DNP3
protocol messages specify source and destination addresses. A destination address specifies which
device should process the data, and the source address specifies which device sent the message.
Having both destination and source addresses satisfies at least one requirement for peer-to-peer
communication since the receiver knows where to direct a response. Each device supporting the DNP
protocol must have a unique address within the collection of devices sending and receiving DNP
messages.
22.1.6.1. Address Learning for DNP
The RX1500 implements both local and remote address learning for DNP.
A local Device Address Table is populated with DNP Addresses learned for local and remote DNP
devices. Each DNP address is associated with either a local serial port or a remote IP address.
When a message with an unknown DNP source address is received on a local serial port, the DNP
source address and serial port number are entered into the Device Address Table. When a message
with an unknown DNP source address is received from the IP network, on the IP interface that is
configured as the DNP learning interface, the DNP source address and the IP address of the sender
are entered into the Device Address Table.
When a message with an unknown DNP destination address is received on a local serial port, the
message is sent in a UDP broadcast out the network interface configured as the DNP learning interface.
When a message with an unknown DNP destination address is received from the IP network, it is sent
to all local serial ports configured as DNP ports.
UDP transport is used during the DNP address learning phase.
All learned addresses will be kept in the Device Address Table, which is saved in non-volatile memory,
which makes it unnecessary to repeat the DNP address learning process across a RX1500 reboot or
accidental power loss.
An aging timer is maintained per DNP address in the table, and is reset whenever a DNP message is
sent to or received for the specified address.
This learning facility makes it possible to configure the DNP3 protocol with a minimum number of
parameters: a TCP/UDP port number, a learning network interface and an aging timer.
22.1.6.2. DNP Broadcast Messages
DNP addresses 65521 through 65535 are reserved as DNP3 broadcast addresses. The RX1500
supports DNP3 broadcast messages. DNP broadcast messages received on local serial ports are
transmitted to all IP Addresses in the DNP Device Address Table (whether learned or statically
configured). When a DNP broadcast message is received from the IP network, it is transmitted on all
local serial ports configured as DNP ports.
