PGR-6150 Motor Protection System
Rev. 1
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Pub. PGR-6150-M. September 29, 2010 • www.littelfuse.com • © 2010 Littelfuse • POWR-GARD
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11. MODBUS RTU PROTOCOL
This document describes the steps to follow to read and write data to the PGR-6150, using
Modbus RTU protocol. The memory map is described in Section 11.8.
Only the master can initiate a message transaction. Messages can be addressed to individual
slaves or they can be broadcast messages. Broadcast messages are executed on the slaves
but unlike individually addressed messages, the slaves do not generate a reply. To broadcast a
message, the 0 address is used.
Each byte of data is transmitted asynchronously and is made up of: 1 start bit, 8 data bits and
1 stop bit. The speed is 19,200 bit/s.
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 completed
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-6150 are delayed by 3.5 characters.
The PGR-6150 Modbus protocol supports the following functions
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Read Holding Registers (Function Code 3).
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Write Multiple Registers (Function Code 16).
The following table shows the generic package format that is valid for transmission and
reception. However, each function has its own characteristics, as will be described further on.