43
6 Ethernet protocols
Example:
reading n words
Reading the measured value from the first analog input (these are the first 2
words starting from Modbus address 0x1257).
See also the Modbus example in Chapter 4.6.2 Read n words, Page 20.
Query:
Response:
MBAP header Modbus telegram
(without slave address and CRC)
00 01 00 00 00 06 FF 03 12 57 00 02
2 bytes
Transaction
ID
2 bytes
Protocol ID
2 bytes
Length
1 byte
Unit ID
1 byte
Function
code
2 bytes
Address of
first word
2 bytes
Number of
words
Assignment
of response
to request
(consecu-
tive num-
bering)
For Mod-
bus always
0x00
Length of
the request
in bytes
starting with
(incl.) "Unit
ID"; 6 bytes
here (0x06)
With TCP
always
0xFF (ex-
cept for
broadcast)
Function
code for
"Reading n
words"
First word
of analog
inputs to
be read
2 words
should be
read
MBAP header Modbus telegram
(without slave address and CRC)
00 01 00 00 00 07 FF 03 04 19 99 43 48
2 bytes
Transaction
ID
2 bytes
Protocol ID
2 bytes
Length
1 byte
Unit ID
1 byte
Function
code
1 byte
Number of
bytes read
4 bytes
Data read
Assignment
of response
to request
(consecu-
tive num-
bering)
For Mod-
bus always
0x00
Length of
the re-
sponse in
bytes start-
ing with (in-
cl.) "Unit
ID"; 7 bytes
here (0x07)
With TCP
always
0xFF (ex-
cept for
broadcast)
Function
code for
"Reading n
words"
4 bytes
were read
Measured value
200.1
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