Instruction Manual

28 Communications Specification
Chapter 2: Modbus-RTU Protocol
response is returned. This function is the only one that
brings the port out of Listen Only Mode. If the port is not
currently in Listen Only Mode, a normal response is
returned. This occurs before the restart is executed.
Subfunction Data Field (Query) Data Field (Response)
x00 01 x00 00 Echo Query Data
x00 01 xFF 00 Echo Query Data
x02 Return Diagnostic Register
• The contents of the slave’s 16-bit diagnostic register are
returned in the response.
Subfunction Data Field (Query) Data Field (Response)
x00 02 x00 00 Diagnostic Register
Contents
x04 Force Listen Only Mode
• Forces the addressed slave to its Listen Only Mode for
Modbus-RTU communications. This isolates it from the
other devices on the network, allowing them to continue
communicating without interruption from the addressed
slave. No response is returned.
• When the slave enters its Listen Only Mode, all active com-
munication controls are turned off. The ready watchdog
timer is allowed to expire, locking the controls off. While in
this mode, any Modbus-RTU messages addressed to the
salve or broadcast are monitored, but no actions will be
taken and no responses will be sent.
• The only function that will be processed after the mode is
entered will be the Restart Communications Option func-
tion (function code 8, subfunction 1).
Subfunction Data Field (Query) Data Field (Response)
x00 04 x00 00 No Response
Returned
x0A Clear Counters
• Clears all Communication Event counters. Counters are
also cleared upon power-up.
Subfunction Data Field (Query) Data Field (Response)
x00 0A x00 00 Echo Query Data
x0B Return Bus Message Count
• The response data field returns the quantity of messages that
the slave has detected on the communications system since
its last restart, clear counters operations, or power-up.
Subfunction Data Field (Query) Data Field (Response)
x00 0B x00 00 Total Message Count
x0C Return Bus Communication Error Count
• The response data field returns the quantity of CRC errors
encountered by the slave since its last restart, clear counters