Sel SEL-421 - Mb8; Message Transmission

R.5.16
SEL-421 Relay Reference Manual Date Code 20111215
SEL Communications Protocols
SEL MIRRORED BITS Communications
➢ If you set MBNUM to less than 8, you can use the
remaining channels (up to a total of eight) for the
features listed below.
➤ Message and Time Synchronization. If MBNUM is less than 8,
the relay dedicates a logical data channel to message framing
and time synchronization.
➤ Analog Channels. Setting MBNUMAN controls the number of
Analog Channels.
➢ If MBNUM := 8, all channels are used for logic bits and
MBNUMAN is forced to 0.
➢ If MBNUM := 7, seven channels are used for logic bits
and one channel is used for message and time
synchronization.
➢ If MBNUM is less than 7, you can use the remaining
channels for analog channels by setting the desired
number of channels in MBNUMAN (1 to
7–MBNUM).
➤ Virtual Terminal Sessions. Setting MBNUMVT controls the
number of additional channels available for the virtual terminal
session.
➢ If MBNUMVT := OFF, the relay does not dedicate any
additional channels to the virtual terminal session.
➢ If there are spare channels (7 – MBNUM –
MBNUMAN > 0), you can use MBNUMVT to
dedicate these additional channels to the virtual
terminal session.
The virtual terminal session uses channels differently than other data
exchange mechanisms. There can be only one active virtual terminal session
across a M
IRRORED BITS link. One channel, included in the synchronization
data, is always dedicated to this virtual terminal session. If you assign
additional channels to the virtual terminal session (set MBNUMVT > 0), you
will improve the performance of the virtual terminal session. The relay uses
the additional channels to exchange data more quickly.
Operation
MB8
While the SEL-421 does not have a setting for the MB8 protocol implemented
in some SEL products, you can configure the relay to communicate with SEL
devices set to MB8A or MB8B. Set the SEL-421 protocol setting PROTO to
MBA or MBB. Set the STOPBIT setting to 2. Set all other settings to match
those in the other device.
Message Transmission
Depending on the settings, the SEL-421 transmits a MIRRORED BITS
communications message every 1/8 to 1/2 of an electrical cycle (see
Tab le 5.17). Each message contains the most recent values of the transmit bits.
If you enabled any of the extended features through the settings, note that the
relay transmits a portion of the extended data in each message.