Table of Contents
Advertisement
3. MODBUS PROTOCOL
3.4.2 Read Holding Registers (Reading data in holding registers: 03h)
Data in consecutive registers is read for the specified number of data points starting from the specified
register address.
(1) Message frame
Query Message
Slave Address
Function
(8 bits)
03h
Message
Slave Address
Function
Starting Address (Note 2)
No. of Points
CRC Check
Response Message
Slave Address
Function
(8 bits)
(8 bits)
Message
Slave Address
Function
Byte Count
Data (Note 1)
CRC Check
Note 1. Higher 8 bits of 1-byte data are set to "0h" when this data is returned. To use the signed 1-byte data as 2-byte data, perform
sign extension on the master (controller) side.
2. Registers can be classified into two types: registers that can be continuously accessed and particular registers that cannot be
continuously accessed.
To read particular registers, read only the target registers.
For the details on whether the target registers can be continuously accessed or not, refer to the directions for use of each
function of the Modbus registers described in chapter 4.
Starting Address
H
L
(8 bits)
(8 bits)
Size
Set a station number to which the message is sent.
8 bits
The number "0" (station number for broadcast sending) cannot be set.
8 bits
Set "03h".
16 bits
Set a start address of the holding registers to read.
Set the number of points of data to read starting from the start address of the
holding registers from which data is read.
16 bits
Set the number of read points described in the list of holding registers.
To read the data in continuous registers, set a value obtained by adding the
number of read points of the target registers to this number.
Data for CRC error check
16 bits
This data is calculated automatically by a controller.
Byte Count
H
(8 bits)
(8 bits)
Size
8 bits
The station number of a slave (servo amplifier) is returned.
8 bits
When the message was received without errors, "03h" is returned.
The Data frame size (in units of bytes) is returned.
8 bits
A value obtained by multiplying the value set for No. of Points in the Query
Message by 2 is returned.
Data starting from the start address specified in the Query Message is returned.
16 bits × n
Data is read in order of H (higher bits) and L (lower bits).
Data is read in order starting from the start address.
Data for CRC error check
16 bits
This data is calculated automatically by a servo amplifier, and the result of the
calculation is returned.
No. of Points
H
(8 bits)
(8 bits)
Description
Data
L
to
(8 bits)
to
(8 bits)
Description
3 - 4
CRC Check
L
L
(8 bits)
CRC Check
H
L
L
(8 bits)
(8 bits)
H
(8 bits)
H
(8 bits)
Advertisement
Table of Contents
