Date Code 20010518 Appendix I I-1
SEL-2020 Instruction Manual
APPENDIX I: DISTRIBUTED NETWORK PROTOCOL (DNP)
V3.00
OVERVIEW
The SEL-2020 supports DNP V3.00 Level 2 protocol on Port 16. It can be used for data access
and for control. For a complete description of this protocol refer to the DNP Basic Four
Documentation Set and the DNP V3.00 Subset Definition, both of which are available from the
DNP User’s Group.
INSTALLATION
If you are using the SEL-2020 in a point-to-point DNP connection, simply connect Port 16 to
your DNP master. If you are using the SEL-2020 in a multi-drop configuration, you will need to
connect a transceiver to Port 16. If the transceiver has a Carrier Detect signal, connect it to Port
16’s CTS input. Otherwise connect RTS to CTS at Port 16. If the transceiver requires a signal to
enable its transmitter, the RTS output on Port 16 can be used for that function.
CONFIGURATION
Section 6: Settings lists all of the DNP related settings and their functions. In order to optimally
configure the SEL-2020 for DNP operation, you will need to understand the basics of DNP and
the capabilities of your DNP master.
Data-Link Operation
There are two important decisions you need to make about your data-link layer operation. One is
how you want to handle data-link confirmation. The other is how you want to handle data-link
access.
If you have a highly reliable communication link, you can disable data-link confirmation
altogether, which significantly reduces communications overhead. Otherwise, you will need to
enable confirmation, determine how many retries you want to allow, and what the data-link
timeout should be. The noisier your communications channel is, the more likely it is that a
message will be corrupted. Thus, you should set your number of retries higher on noisy
channels. Set the data-link timeout long enough to allow for the worst-case response of your
master plus transmission time.
When the SEL-2020 decides to transmit on the DNP link, it has to wait if the physical connection
is in use. The SEL-2020 monitors physical connections by using both its CTS input (treated as a
carrier detect) and by monitoring character receipt. Once the physical link goes idle, as indicated
by CTS being deasserted and no characters being received, the SEL-2020 will wait a configur-
able amount of time before beginning a transmission. This hold-off time will be a random time
between the MIN_DELAY and MAX_DELAY setting values. This hold-off is random so
multiple devices waiting to communicate on the network will not continually collide.
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