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Appendix G
Modbus Protocol Illustration
Response
The slave responds with its address, function code, number
of data bytes, and the data. The contents of the registers
requested (data) are two bytes each. The first byte includes
the high order bits and the second, the low order bits.
High
Slave
Function
Byte
Order
Address
Code
Count
Data
08h
03h
04h
00h
Figure G4 – Read Holding Register Response Message
Register 40006 , Normal Voltage Ph1-Ph2, has a value of
118 (76 hex) and register 40007, Normal Voltage Ph2-
Ph3 has a value of 120 (78 hex).
Write Single Coil (Function Code 05)
This function forces a single coil either ON or OFF . A value
of 65,280 ( FF 00 Hex) will set the coil ON and the value
zero will turn it OFF ; all other values are illegal and will not
affect that coil.
Query
Figure G5 is an example of a request to slave number 3
to turn ON coil 71.
Starting
Starting
Slave
Function
Address
Address
Address
Code
High
Order
03h
05h
00h
Figure G5 – Write Single Coil Query Message
Response
The slave's normal response to the Write Single Coil
query is to return the original message after the coil state
has been altered.
Starting
Starting
Slave
Function
Address
Address
Address
Code
High
Order
03h
05h
00h
Figure G6 – Write Single Coil Response Message
Write Single Holding Register (Function Code 06)
This function allows the master to modify the contents
of one holding register.
Query
Figure G7 is an example of a request to preset register
40041 (Normal Pickup Voltage) to 92 (00 5 C hex) in
slave device 17.
20
MX150/MX250 Series Modbus Network Card (71R-2200)
I
(cont'd)
Low
High
Low
Error
Order
Order
Order
Check Field
Data
Data
Data
(LRC or CRC)
76h
00h
78h
—
Error
Data
Data
Check Field
High
Low
Low
(LRC or
Order
Order
Order
CRC)
46h
h
00h
—
FF
Error
Data
Data
Check Field
High
Low
Low
(LRC or
Order
Order
Order
CRC)
46h
h
00h
—
FF
(cont'd)
Starting
Starting
Slave
Function
Address
Address
Address
Code
High
Order
Order
11h
06h
00h
Figure G7 – Write Single Holding Register Query Message
Response
The slave's response to the Write Single Holding
Register query is to return the original message after the
registers have been altered.
Starting
Starting
Slave
Function
Address
Address
Address
Code
High
Order
Order
11h
06h
00h
Figure G8 – Write Single Holding Register Response Message
Write Multiple Coils (Function Code 15)
Forces each coil in a sequence of coils to either ON or
. The requested ON / OFF states are specified by con-
OFF
tents of the query data field. A logical '1' in a bit position
of the field requests the corresponding coil to be ON and a
logical '0' requests it to be OFF . Coils are addressed
starting at 0. For examples coil 1 is addressed as 0.
Query
The following example is a request to force a series of six-
teen coils starting at coil 41 (addressed as 40, or 28 hex)
in slave device 9.
The query data contents consist of two bytes: 3 C 9 B hex
(0011 1100 1001 1011 binary). The binary bits corre-
spond to the coils in the following way:
Bit:
0
0
1
1
1
1
Coil:
48 47 46 45 44 43 42 41
The first byte sent (3 C hex) addresses coils 41-48, with
the least significant bit addressing coil 41. The second
byte sent (9 B hex) addresses coils 49-56, with the least
significant bit addressing coil 49.
Starting
Starting
Number
Slave
Function
Address
Address
of Coils
Address
Code
High
Low
High
Order
Order
Order
09h
0Fh
00h
28h
00h
Figure G9 – Write Multiple Coils Query Message
Error
Data
Data
Check Field
High
Low
Low
(LRC or
Order
Order
CRC)
28h
00h
5 C h
—
Error
Data
Data
Check Field
High
Low
Low
( LRC or
Order
Order
CRC
28h
00h
5 C h
—
0
0
1
0
0
1
1
0
1
56 55 54 53 52 51 50 49
Number
Error
Data
Data
of Coils
Byte
Check
High
Low
Low
Count
Field
Order
Order
Order
( LRC or RC )
10h
02h
3 C h
9 B h
ABB Zenith
)
1
–
I
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