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8. Modbus protocol
8. 6. Diagnostics (8)
Request and response:
The counters are unsigned words.
Example: Values 16#31 and 16#32 echoed by slave 4
Request and response:
Slave no. 08 Sub-code Data CRC16
Hi Lo Hi Lo Hi Lo
1 byte 1 byte 2 bytes 2 bytes 2 bytes
Sub
code
Function Request
data
Response
data
00 Echo
This function asks the slave to return the request sent by the master. The size
of the data is limited to 2 bytes.
XX YY XX YY
01 Reinitialize channel
This function is used to reinitialize slave communication and, in particular, to
make it exit listen only mode (LOM).
16#00 00
or
16#FF 00
16#00 00
or
16#FF 00
03 Change ASCII delimiter
In ASCII mode, messages are delimited by the line feed character
(LF = H'0A). This function is used to change this character.
XX 00
XX = new delimiter
XX 00
04 Change to LOM mode
This function is used to force a slave to listen only mode (LOM). In this mode,
the slave does not handle messages which are addressed to it, nor does it send
any responses, except when the channel is reinitialized.
00 00 00 00
0A Counter reset
This function resets all the counters responsible for monitoring slave
exchanges.
00 00 00 00
0B Read the number of correct messages on the line without checksum error 00 00 Value of
counter
0C Read the number of incorrect messages on the line with checksum error 00 00 Value of
counter
0D Read the number of exception responses sent by the slave 00 00 Value of
counter
0E Read the number of messages addressed to the slave excluding
broadcast messages regardless of type
00 00 Value of
counter
0F Read the number of broadcast messages on the line regardless of type 00 00 Value of
counter
10 Read the number of NAK exception responses
The value read is always 0.
00 00 00 00
11 Read the number of slave not ready responses
The value read is always 0.
00 00 00 00
12 Read the number of messages received with character overrun 00 00 Value of
counter
Slave no. Code Subcode Value of
1
st
byte
Value of
2
nd
byte
CRC CRC
Hi Lo Lo Hi
04 08 00 00 31 32 74 1B