G-18 Distributed Network Protocol (DNP) 3.00 Date Code 20010731
SEL-352-1, -2 Instruction Manual
Modem Support
The modem handling will only be applied when the port settings include the following:
PROTOCOL = DNP
MODEM = Y
On power-up and settings change, the relay shall initialize the modem by issuing the string
"+++AT" followed by the MSTR string and <CR>. This will initialize the modem. The MSTR
(modem string) is a port setting visible only when the protocol setting is DNP. The MSTR
setting is a series of ASCII characters that initialize the modem by sending the modem a series of
commands.
If someone calls in, the modem will send "RING" and "CONNECT" messages to the relay.
These messages, as well as all messages received while DCD is low, shall be ignored. All DNP
messages received while connected shall be treated normally.
If the relay needs to send an unsolicited message and it is not currently connected, it must
attempt to make a connection by sending the string "+++ATDT" followed by the phone number
and <CR>. It shall then wait for a "CONNECT" message. Once "CONNECT" is received and
CTS is asserted, the relay can consider itself connected and continue its transaction. If
connection is not achieved within MDTIME seconds of initiating the phone call, the relay shall
issue the command "+++ATH<CR>" and wait at least MDRETI seconds before trying again and
try MDRETN times before giving up. If it fails to connect in the first try, it will try again at a
later time every six hours.
If the relay initiates a connection, it shall disconnect once there have been no transactions for
TIMEOUT time, using the disconnect command "+++ATH<CR>". Also, if an outside caller
connects to the modem in the SEL-352 Relay, the SEL-352 Relay will disconnect the modem if
there have been no transactions for the TIMEOUT time.
Note 1
: Because of the connection requirements described here, it will not be possible to use
hardware flow control (RTSCTS) with the modem. This means that it is important to
select a port baud rate low enough that the modem connection will not end up slower,
or there will be a high likelihood of losing characters.
Note 2
: The CTS signal shall be treated as a data carrier detect (DCD). This means that the
message may only be transmitted while DCD is asserted. (Normally, a modem will be
connected with a C220 or C222 cable that ties the DCD of the modem to the CTS.)
Virtual Terminal
The purpose of this Virtual Terminal (VT) Protocol is to allow ASCII data transfers between a
master and an SEL relay over a DNP port. DNP 3.00 objects 112 and 113 are used for
embedding the ASCII communications over the DNP port. At the master each slave channel is
assigned a Virtual Port number. Only one channel, with a Virtual Port number of "0" (for
ASCII), is supported in the relay.
Object 112 is used with the Function code Write (FC=2) to send data from the Master side to the
Slave side (IED) of the link.
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