PGR-8800 Arc Flash Relay
Rev. 2
Pub. PGR-8800-M. December 14, 2012 • www.littelfuse.com • ©
2012
Littelfuse • •
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APPENDIX A PGR-8800 MODBUS RTU PROTOCOL
A.1 PROTOCOL
The MODBUS RTU protocol is implemented in the PGR-8800, and allows up to four modules to be
connected using a 2-wire TIA-485 bus. The MODBUS interface will provide status information for
sensors, analog inputs, digital inputs, digital outputs, and numerous event and error flags. It also
provides the ability to remotely reset the PGR-8800. Writes are not allowed by default.
A.1.1 Configuration Setup
A.2 MESSAGE
SYNCHRONIZATION
Message synchronization is accomplished by detection of an idle communication line. The
communication line is considered idle when no communication exists for an equivalent delay of 3.5
characters.
The first byte received after idle-line detection is interpreted as the address byte of the next message.
Message bytes must be transmitted in a continuous stream until the complete message has been
sent. If a delay of more than 3.5 characters exists within the message, the message is discarded.
Response messages from the PGR-8800 are delayed by at least 3.5 character delays.
A.3 ERROR
CHECKING
Modbus RTU uses a 16-bit cyclic redundancy check (CRC). The error check includes all of the
message bytes, starting with the first address byte. When a CRC error is detected, the message is
discarded and there will be no response.