Date Code 20001006 MIRRORED BITS™ I-1
SEL-351 Instruction Manual
APPENDIX I: MIRRORED BITS™ (IN FIRMWARE VERSIONS
1 TO 4)
OVERVIEW
MIRRORED BITS is a direct relay-to-relay communications protocol which allows protective relays
to exchange information quickly and securely, and with minimal expense. The information
exchanged can facilitate remote control, remote sensing, or communications-assisted protection
schemes such as POTT, DCB, etc. The SEL-351 supports two MIRRORED BITS channels,
differentiated by the channel specifiers A and B. Bits transmitted are called TMB1x through
TMB8x, where x is the channel specifier (e.g., A or B), and are controlled by the corresponding
SELOGIC Control Equations. Bits received are called RMB1x through RMB8x and are usable as
inputs to any SELOGIC Control Equations. Channel status bits are called ROKx, RBADx,
CBADx and LBOKx and are also usable as inputs to any SELOGIC Control Equations. Further
channel status information is available via the COM command.
Important: Do not connect an unconfigured port to a MIRRORED BITS device. Otherwise the
relay will appear to be locked up. Configure the port first, then connect the device.
OPERATION
Message Transmission
All messages are transmitted without idle bits between characters. Idle bits are allowed between
messages.
• At 4800 baud, one message is transmitted each 1/2 power system cycle.
• At 9600 baud, one message is transmitted each 1/4 power system cycle.
• At 19200 and 38400 baud, one message is transmitted each 1/8 power system cycle for
the SEL-321 and 1/4 power system cycle for the SEL-351.
Message Decoding and Integrity Checks
The relay will deassert a user-accessible flag per channel (hereafter called ROKx) upon failing any
of the following received-data checks:
• Parity, framing, or overrun errors.
• Receive data redundancy error.
• Receive message identification error.
• No message received in the time three messages have been sent.
Courtesy of NationalSwitchgear.com
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