Modbus communication, 1 general, 2 message construction – Yokogawa EJX930A User Manual
Page 32: 3 broadcast, 4 function code, 1 01 (0x01) read coils, Modbus communication -1, General -1, Message construction -1, Broadcast -1
<5. Modbus Communication>
5-1
IM 01C25R05-01EN
5. Modbus Communication
5.1 General
EJX Multivariable transmitter can communicate with
the Modbus host.
Table 5.1
Modbus specification
Item
Description
Communication
protocol
2-wire half duplex RS-485 Modbus
Flow control
None
Baud rate
1200, 4800, 9600*, 19200
Start bit
1 bit (Fixed)
Stop bit
1 bit*, 2 bits
Parity bit
Odd, Even, None*
Transfer mode RTU (Remote Terminal Unit)
Data length: 8 bits
LSB (Least significant bit sent first)
Slave address
1* to 247
Support
function
1: Read Coils
2: Read Discrete Inputs
3: Read Holding Registers
4: Read Input Register
5: Write Single Coil
8: Diagnostic (00: Return Query Data)
16: Write Multiple registers
43: Read device Identification
Max
transmission
frame size
200 byte
Turnaround
Delay Time
10* to 200 ms
*:
Factory default setting
5.2 Message construction
The message transmitted from host is constructed
as below.
#
Name
Size
Contents
1 Slave
address
1 byte The number host system
distinguish the slave device
(1 to 247)
2 Function
code
1 byte The function code from host
3 Data
0 to
196
bytes
Specify register address,
number or data according to
the function code.
4 Error check
2 byte cyclic redundancy check
(CRC-16)
5.3 Broadcast
Broadcast is a function receiving a command from
all devices connected under multi drop mode.
(1) Broadcast is executed by setting 0x00 to slave
address.
(2) This message function is executed regardless
of device slave address.
(3) This message is used only for write function.
(4) No response is returned from the slave device.
5.4 Function code
5.4.1 01 (0x01) Read Coils
Specified quantity of Coil contents is read from
Specified Starting Address.
Not corresponding to Broadcast.
Request
Items
Size [byte]
Data
Address
1
1 to 247
Function code
1
0x01
Starting Address
2
0x0000 to 0xFFFF
Quantity of coils
2
-
CRC
2
-
Response
Items
Size [byte]
Data
Address
1
1 to 247
Function code
1
0x01
Byte count
1
N
*1
Coil Status
*2
N
*1
-
CRC
2
-
*1:
N is 1 when quantity of coils is less than or equal to 8.
*2:
e.g. When reading address is 1000 – 1007, MSB is 1007
and LSB is1000.