G-20
Distributed Network Protocol (DNP) 3.00
Date Code 20020129
SEL-387E Instruction Manual
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 3.0 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.
Object 113 is used to send data from the relay side to the Master side of the link. Master devices
may use only Function codes Read (FC=1). The relay uses only Function codes Response
(FC=129).
The procedure for accessing these objects is as follows. Master devices transmit data to relay
devices by writing one or more of object 112 to a relay using the Virtual Port number as the DNP
point number. Relays send information to the Master using the Virtual Port number by
responding to a Master READ (FC=1) request of object 113. Messages can flow in either
direction at any time, however the relay sends messages only at the request of the Master. There
are no explicit procedures for the initiation or conclusion of a VT session (i.e., implicit
connections exist by the mere presence of a VT-compatible Slave IED).
Virtual terminal supports all ASCII commands listed in the
Command Summary
at the end of
this instruction manual. You do not need a password to login to a virtual terminal session
through a DNP port, but you will need the appropriate access levels for setting changes and
breaker operations. A virtual terminal session times out in the same way as an ASCII session.
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