Date Code 20170927 Instruction Manual SEL-751 Relay
Appendix J
MIRRORED BITS Communications
Overview
MIRRORED BITS is a direct relay-to-relay communications protocol that allows
IEDs to exchange information quickly, securely, and with minimal expense.
Use M
IRRORED BITS for functions such as remote control and remote sensing.
The SEL-751 Feeder Protection Relay supports two M
IRRORED BITS
communications channels, designated A and B. Use the port setting PROTO to
assign one of the M
IRRORED BITS communications channels to a serial port;
PROTO:=MBA for M
IRRORED BITS communications Channel A or
PROTO:=MBB for M
IRRORED BITS communications Channel B. MIRRORED
B
ITS are either Transmit MIRRORED BITS (TMB) or Received MIRRORED BITS
(RMB). Transmit M
IRRORED BITS include TMB1A–TMB8A (channel A) and
TMB1B–TMB8B (channel B). The last letter (A or B) designates with which
channel the bits are associated. Received bits include RMB1A–RMB8A and
RMB1B–RMB8B. Control the transmit M
IRRORED BITS in SELOGIC control
equations. Use the received M
IRRORED BITS as arguments in SELOGIC control
equations. The channel status bits are ROKA, RBADA, CBADA, LBOKA,
ROKB, RBADB, CBADB, and LBOKB. You can also use these channel
status bits as arguments in SEL
OGIC control equations. Use the COM
command (see Section 7: Communications) for additional channel status
information.
Because of different applications, the SEL product range supports several
variations of the M
IRRORED BITS communications protocol. Through port
settings, you can set the SEL-751 for compatible operation with SEL-300
series devices, SEL-2505 Remote I/O Modules, and SEL-2100 Logic
Processors. When communicating with an SEL-400 series relay, be sure to set
the transmission mode setting in the SEL-400 series relay to paced
transmission (TXMODE := P).
Operation
Message
Transmission
In the SEL-751, the MIRRORED BITS transmission rate is a function of both the
data rate and the power system cycle. At data rates slower than 9600, the
SEL-751 transmits M
IRRORED BITS as fast as possible for the given rate. At
rates of 9600 bps and faster, the SEL-751 self-paces, using a technique similar
to the SEL-400 series pacing mode. There are no settings to enable or disable
the self-pacing mode; the SEL-751 automatically enters the self-pacing mode
at data rates of 9600, 19200, and 38400. Table J.1 shows the transmission
rates of the M
IRRORED BITS messages at different data rates.
IMPORTANT: Be sure to configure
the port before connecting to a
M
IRRORED BITS device. If you connect
an unconfigured port to a M
IRRORED
B
ITS device, the device will appear to
be locked up.
NOTE: Complete all of the port
settings for a port that you use for
M
IRRORED BITS communications before
you connect an external M
IRRORED BITS
communications device. If you
connect a M
IRRORED BITS
communications device to a port that
is not set for M
IRRORED BITS
communications operation, the port
will be continuously busy.
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