Chapter 6. Serial I/O, SNP & RTU Protocols
244 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD
Response:
â–ª See descriptions for individual Diagnostic Codes.
Diagnostic Return Query Data Request (Loopback/Maintenance Code 00):
â–ª An address of 0 is not allowed for the return query data request.
â–ª The values of the two Data field bytes in the query are arbitrary.
â–ª The normal response is identical to the query.
â–ª The values of the Data bytes in the response are equal to the values sent in the query.
Diagnostic Initiate Communication Restart Request (Loopback/Maintenance Code 01):
â–ª An Address of 0 indicates a broadcast request. All slave stations process a broadcast request and no
response is sent.
â–ª This request disables the listen-only mode (enables responses to be sent when queries are received
so that communications can be restarted).
â–ª The value of the first byte of the Data field (DATA1) must be 0 or FF. Any other value will cause an
error response to be sent. The value of the second byte of the Data field (DATA2) is always equal to
0.
â–ª The normal response to an Initiate Communication Restart query is identical to the query.
Diagnostic Force Listen-Only Mode Request (Loopback/Maintenance code 04):
â–ª An Address of 0 indicates a broadcast request. All slave stations process a broadcast request.
â–ª After receiving a Force Listen-Only mode request, the RTU device will go into the listen-only mode,
will not perform a requested function, and will not send either normal or error responses to any
queries. The listen-only mode is disabled when the RTU device receives an Initiate Communication
Restart request or when the RTU device is powered up.
â–ª Both bytes in the Data field of a Force Listen-Only Mode request are equal to 0. The RTU device never
sends a response to a Force Listen-Only Mode request.
Note: Upon power-up, the RTU device disables the listen-only mode and is enabled to continue
sending responses to queries.
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