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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11.1.
Modbus Package Format
Slave
Address
1 byte
Each device on a communications bus must have a unique
address.
All the relay ports shall use this address which can be set at a
value of between 1 and 247. Slave address 0 indicates a
broadcast message. All the communications bus slaves shall
perform the requested action, but none shall reply to the master.
The Broadcast shall only be accepted for writing.
Function
Code
1 byte
Function codes supported are 3 for reading and 13 for writing.
When the slave must reply with an exception to one of these
messages, this is indicated by setting the most significant bit of
the corresponding function to 1.
An exception for function 3 shall be indicated with 83 as a
function code, and an exception for function 16 or 0x10 in
hexadecimal, shall be indicated with 0x90.
Data
N bytes
Data consists of a variable number of bytes, depending on the
function code.
It can include: Addresses, data lengths, settings, exception
codes or commands.
CRC
2 bytes
Two byte CRC code.
Modbus RTU includes a 16 bit cyclic redundancy check (CRC) in
each message, for error detection. If the slave detects an
erroneous message, based on an incorrect CRC, it will not
perform any actions, nor will it reply to the master.
The CRC bit order is LSB-MSB.
Dead time
Time
required to
transmit 3.5
bytes
A message is considered terminated when no data is received
for a period of 3.5 bytes:
15 ms at 2,400 bps
2 ms at 19,200 bps…etc.
11.2. Function Codes
HEX
DEC CODE
Modbus Name
Definition
Comments
0x03
3
Read Holding
Registers
Reading of any value
This function allows the master to read 1
or more consecutive registers of a relay.
The registers are always 16 bits, with the
most significant byte first.
A maximum of 60 registers can be read in
a single READ.
0x10
16
Preset Multiple
Registers
Writing
This function allows one or more registers
to be written representing one or more
settings.
The registers have 2 byte length values,
transmitted with the most significant byte
first.
A maximum of 60 registers can be written
in a single WRITE.