Schweitzer Engineering Laboratories SEL-2890, Instruction Manual

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Date Code 20210715 SEL-2890 Instruction Manual
Introduction & Specifications
Applications
1.7
Telnet Tunnel
An automatic, fixed point-to-point serial connection across an Ethernet network is
established with a Telnet tunnel. Use this method when you need a continuous point-to-
point connection for serial protocols like Modbus
®
and DNP3. The settings for this
tunnel, IP and Port, are saved in nonvolatile memory and will be retained if power is
cycled on the SEL-2890. The SEL-2890 automatically connects to the remote device
(it may be another SEL-2890) and allows serial traffic whenever the Ethernet network
is operational.
TCP Tunnel
A TCP tunnel functions almost identically to a Telnet tunnel except that the network
connections are opened and closed rather than maintained continuously.
BTCP Tunnel
A BTCP (broadcast TCP) tunnel functions like a multipoint serial network. Figure 1.3
shows the physical connection for this type of tunnel service. The Telnet tunnel
establishes a connection and then maintains a connection. By contrast, the BTCP
tunnel is established between two SEL-2890 transceivers, then data are transmitted and
received, and lastly the connection is terminated. The ability to use this type of tunnel
for broadcast messages is the important aspect. All devices must be on the same
subnet.
As an example, the master IED device in Figure 1.3 sends out, via a serial port, a
request for data from Slave #1. These serial data are then transmitted to the Ethernet
network through the SEL-2890 connected to the master polling IED. All of the other
SEL-2890 transceivers receive the request and send it through their respective serial
ports. However, only Slave #1 answers and the data are returned to the master polling
IED.
It is important to note that the serial protocol described in this method uses an address
system. This address system is common in virtually all SCADA type polling protocols.
If a non-addressed, serial protocol is used, all of the slaves will answer and the master
polling IED will receive the data but may not know which slave it came from. This may
be acceptable if the serial data are self-identifying.
Collisions will occur if all the slave devices answer at the same time. The SEL-2890
transceiver’s Ethernet processor will make the necessary adjustments and will keep
retransmitting until a message is successfully sent. However, the network may be
flooded by these retries and network performance will be significantly degraded.
The broadcast method is defined in the Ethernet standard. See your network
administrator about using broadcast messages in your network. Details on setting the
SEL-2890 for BTCP tunneling using broadcast are furnished in
Section 3: Settings and
Commands , followed by an example in Section 4: Detailed Application Examples .