Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

Page: 1302 / 1518

15.32
SEL-400 Series Relays Instruction Manual Date Code 20171006
Communications Interfaces
SEL M
IRRORED
B
ITS
Communication
➤
Logic bits: Setting MBNUM controls the number of channels used
for logic bits, TMB1–TMB8, inclusive.
➢
If you set MBNUM to 8, then you cannot use channels for any of
the following features.
➢
If you set MBNUM to less than 8, you can use the remaining
channels (as many as eight total) 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. It is not guaranteed that multiple analog quantities
will come from the same relay sampling interval.
➢
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).
Note: Analog quantities are converted to Integer values for
transmission via M
IRRORED
B
ITS
. Because of this, they will lose any
fractional value they may have had. To maintain a fixed resolution,
multiply the analog quantity by a set value before transmission, and
divide by the same quantity upon reception. To maintain accuracy,
add 0.5 to the analog quantity after any scaling.
➤
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.
➢
With MBNUM = 7 or less and MBNUMVT = 0, virtual terminal
is still possible because the relay uses the eighth element for
time synchronization and virtual terminal.
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
B
ITS
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
MBG Protocol
The MBG protocol selection allows the user to move the M
IRRORED
B
ITS
Trans-
mit equations to the Group settings for more flexibility in bus transfer schemes.
Using MBG will allow the M
IRRORED
B
ITS
settings to transfer with a Group
Switch when it occurs.