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X02 return diagnostic register, X04 force listen only mode, X0a clear counters – Watlow CLS200, MLS300 and CAS200 User Manual

Page 34: X0b return bus message count, X0c return bus communication error count

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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