R.5.17
Date Code 20111215 Reference Manual SEL-421 Relay
SEL Communications Protocols
SEL MIRRORED BITS Communications
If you have specified virtual terminal data channels for this port, the
designated data channels are normally idle. If you use the PORT command to
open a virtual terminal session for this port and type characters, the relay
transmits these characters through the virtual terminal logical data channels.
Message Reception Overview
When the devices are synchronized and the MIRRORED BITS communications
channel is in a normal state, the relay decodes and checks each received
message. If the message is valid, the relay performs the following operations:
➤ Sends each received logic bit (RMBn) to the corresponding
pickup and dropout security counters, that in turn set or clear
the RMBnc relay element bits.
➤ Accumulates the analog data, and every 18th message, updates
the received analog quantities.
➤ Accumulates the virtual terminal information, and every 18th
message, makes the received character or characters available
to the virtual terminal.
Message Decoding and Integrity Checks
The relay provides indication of the status of each MIRRORED BITS
communications channel, with element bits ROKA and ROKB. During
normal operation, the relay sets the ROKc bit. The relay clears the bit upon
detecting any of the following conditions:
➤ Parity, framing, or overrun errors
➤ Receive data redundancy error
➤ Receive message identification error
➤ No message received in the time three messages have been sent
The relay will assert ROKc only after successful synchronization as described
below and two consecutive messages pass all of the data checks described
above. After ROKc is reasserted, received data may be delayed while passing
through the security counters described below.
While ROKc is not set, the relay does not transfer new RMB data to the
pickup-dropout security counters described below. Instead, the relay sends one
of the user-definable default values to the security counter inputs. For each
RMBn, specify the default value with setting RMBnFL, as follows:
➤ 1
➤ 0
➤ P (to use last valid value)
Individual pickup and dropout security counters supervise the movement of
each received data bit into the corresponding RMBn element. You can set each
pickup/dropout security counter from 1 to 8. A setting of 1 causes a security
counter to pass every occurrence, while a setting of 8 causes a counter to wait
for eight consecutive occurrences in the received data before updating the data
bits. The pickup and dropout security count settings are separate. Control the
security count settings with the settings RMBnPU and RMBnDO.
A pickup/dropout security counter operates identically to a pickup/dropout
timer, except that the counter uses units of counted received messages instead
of time. An SEL-421 communicating with another SEL-421 typically sends
NOTE: c represents the MIRRORED BITS
channel (A or B), n represents the M
IRRORED
B
ITS data channel data number (1–8).
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