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EATON
www.eaton.com
PXR6.1 intelligent controller MODBUS-RTU
communication protocol
1. Protocol overview
1.1. Physical layer
Transmission method: RS485/RS422
Communication address: 0-255
Communication baud rate: 9600BPS, 19200BPS,38400BPS,
115200BPS
Communication distance: Up to 1,000m
Communication media: Shield twisted pair
Communication protocol: MODBUS–RTU
1.2. Link Layer
• Transmission method: host-slave half-duplex mode
First, the signal from the host is addressed to a unique terminal
device (slave device), and then the response signal from the
slave device is transmitted to the host in an opposite direction.
The protocol allows data exchange only between the host and
slave device, rather than between independent slave device.
This will ensure the communication line not to be occupied
during initialization, which is only restricted to respond to query
signals that arrive at the device.
• A data frame format
1 start bit, 8 data, 1 stop bit, and no check bit.
Address
Function code
Data
Check code
8-Bits
8-Bits
N x 8-Bits
16-Bits
• A data packet format
The protocol defines check code and data sequence in great
detail, which are essential for specific data exchange.
Upon arrival at a slave device, the data frame enters into the
addressed device through a simple “port”. The device then
removes the “envelope” (data header) of the data frame, and
reads the data. If no error, the device executes the command
requested by the data. It adds the data it generates into the
obtained “envelope”, and returns the data frame to the sender.
The returned response data contains the slave address (Address),
executed command (Function), requested data (Data) generated
by the executed command, and a check code (Check). The slave
device can identify the error communication from the host and
give different error responses.
1.3. Address filed
A frame is started with an address field, which consists of 8
bits (taken from 1 to 99). These bits indicate the address of the
slave device designated by the user, which will receive data from
the host it connects to. The address of each slave device must
be unique, and only the addressed slave device responds to
the query containing the address. When the slave sends back a
response, the slave address data in the response will tell the host
which slave device it is communicating with.
1.4. Function field
The function field code tells which function is to performed by the
addressed slave device. Table 1–1 lists all function codes, their
meanings and initial functions.
Table 1-1 Function code
Code
Defi nition
Behaviors
3
Read data register
Get the current binary value of one or more
registers
5
DO output control
Place a specifi c binary value into a DO command
register
6
Preset single
register
Place a specifi c binary value in a register
8
Communication
diagnosis
Communication diagnosis
16
Preset multiple
registers
Place specifi c binary values in a series of registers
Sub-function codes of the communication diagnosis function code
08H:
Sub-function code
Function
00
Return the original query data
0A
Reset event register, and the count of sub-function codes 0B
to 12.
0B
Total device information count (Counter 1)
0C
CRC check error count (Counter 2)
0D
Count of error responses issued by the device. (Counter 3)
0E
Correctly counts the requests for communication with this
device. (Counter 4)
0F
Reserved
10
Reserved
11
Reserved
12
Reserved
1.5. Data field
A data field contains the data required by the slave to perform a
specific function or the data collected when the slave responds to
a query. These data may be numerical values, reference addresses
or limiting values. For example, a function field code tells the slave
to read a register, while a data field needs to specify which reg-
ister to start and how many data to read. The embedded address
and data are subject to slave types and the capabilities among
slave devices.
1.6. Error check field
An error check field allows the host and slave to check for errors
during transmission. In some cases, due to electrical noise and
other interferences, a set of data may be changed when trans-
mitted from one device to another. Error checking can ensure
the host or slave not to respond to the data changed during the
transmission, which can improve system safety and efficiency. The
error check adopts a 16-bit cyclic redundancy method.
Instruction Leafl et
Effective September 2022
IZM6 Series Air Circuit Breaker
Operating Instruction Leafl et