Function 08 - Diagnostics (Subfunction 01, Restart Communications Option)
If the LPM is in listen-only mode, the LPM responds to this message by switching out of
listen-only mode (note that no response is sent to the request). If the LPM is not in lis-
ten-only mode, it responds as follows:
Slave address: echoed
Function: 08H
Subfunction high: 00H
Subfunction low: 01H
Query data (16-bit): echoed (0000H or FF00H)
Error check: 16-bit CRC/8-bit LRC
Note that since no Communications Event Log is supported, the 'Query data' field is irrel-
evant (normally FF00H would clear the log).
Function 08 - Diagnostics (Subfunction 04, Force Listen Only Mode)
The LPM responds to this request by switching to listen-only mode. Messages are still
received and parsed, but no responses are transmitted. To switch out of listen-only mode,
issue a 'Restart Communications Option' request (function 08, subfunction 01) or cycle
power.
Function 17 - Report Slave ID
The LPM responds to this request with the following data:
Slave address: echoed
Function: 11H
Byte count: 05H
Slave ID: FFH
Run indicator status: FFH (ON)
Additional data: 'LPM'
Error check: 16-bit CRC/8-bit LRC
Modbus Exceptions
The following standard Modbus exceptions are implemented:
Error code 01 (Illegal Function)
Reported when:
• A function other than 03, 04, 08, or 17 is requested.
• Function 08 is requested, and a subfunction other than 00, 01, or 04 is requested.
Error code 02 (Illegal Data Address)
Reported when:
• Function 03 or 04 is requested and the starting register number is greater than 915 (reg-
ister greater than 30916 or 40916).
Error code 03 (Illegal Data Value)
Reported when:
• Function 03 or 04 is requested and the number of data points is greater than 40 (RTU)
or 20 (ASCII), or the number of data points added to the start address results in a register
greater than 30916 or 40916.
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