R.5.19
Date Code 20111215 Reference Manual SEL-421 Relay
SEL Communications Protocols
SEL MIRRORED BITS Communications
The relay sets RTS to a negative voltage at the EIA-232 connector to signify
that M
IRRORED BITS communications matches this specification. Other relays
may set RTS to a positive voltage at the EIA-232 connector to signify usage of
the R6 version or the R version of M
IRRORED BITS communications.
Settings
The SEL-421 Port settings associated with MIRRORED BITS communications
are shown in Table 5.15 and Tab le 5.16.
Set PROTO := MBA to enable the M
IRRORED BITS communications protocol
Channel A on this port. Set PROTO := MBB to enable the M
IRRORED BITS
communications protocol Channel B on this port.
Setting SPEED:= SYNC (available only on the rear-panel serial ports for
which PROTO:= MBA or MBB) places the serial port in synchronous (or
externally-clocked) mode. The serial port hardware will synchronize transmit
and receive data (TX/RX) to a clock signal applied to the Pin 8 input at any
effective baud rate up to 64000. This setting choice will suit certain
synchronous communications networks.
The relay uses the RBADPU setting to determine how long a channel error
must persist before the relay asserts RBADA or RBADB. The relay deasserts
RBADA and RBADB immediately when it no longer detects a channel error.
The relay uses the CBADPU setting to determine when to assert CBADA and
CBADB. If the short-term channel down time ratio exceeds CBADPU, the
relay asserts the appropriate CBAD bit.
The TXMODE setting provides compatibility with SEL devices that are not
SEL-400 series relays. The SEL-421 can send messages more quickly than the
SEL-300 series relays and other SEL devices can process these messages.
This could lead to loss of data and a failure to communicate properly. When
you set TXMODE to P, the relay sends new M
IRRORED BITS messages every
3 ms even if the selected data speed (SPEED setting) would allow more
frequent messages.
As a function of the settings for SPEED, TXMODE, and MBT, the message
transmission periods are shown in Table 5.17.
NOTE: You must consider the idle
time in calculations of data transfer
latency through a Pulsar MBT
modem system.
Table 5.15 General Port Settings Used With MIRRORED BITS Communications
Name Description Range Default
PROTO Protocol. None, SEL, DNP
a
,
MBA, MBB, RTD,
PMU
a
Optional relay feature.
SEL
MBT Enable Pulsar 9600 modem. Y, N N
SPEED Data speed. Hidden and set to 9600 if
MBT := Y.
300, 600, 1200, 2400,
4800, 9600, 19200,
38400, SYNC
9600
STOPBIT Stop bits. Hidden and set to 1 if
MBT := Y.
1, 2 1
RTSCTS Hardware handshaking enable. Y, N N
NOTE: You must use paced
transmission mode (set
TXMODE := P) when connecting
to an SEL product that is not an
SEL-400 series relay.
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