ProSoft Technology MVI56-AFC - Manual (Page 136)

Events MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 136 of 316 ProSoft Technology, Inc.
February 25, 2011
Error Recovery
During a session, the AFC maintains sufficient context information to accept not
only the next expected Modbus transaction of the sequence but also a repeat of
the previous one, except for the Commit Session and Abandon Session actions
(see below). This is to permit simple repetition of a transaction that has
succeeded from the point of view of the AFC but has failed from that of the host,
which would occur when the Modbus response transmitted by the AFC is not
received by the host. (If the host uses Collapsed Acknowledgement, it must take
care to set the acknowledgement bit correctly in a repeated Fetch.) Other than in
such a case, any Modbus transaction that does not comply strictly with the
conditions and sequencing described above is rejected with a Modbus exception
code (typically code 2, Illegal Address, if not addressing LDW offset 0, otherwise
code 3, Illegal Data).
Because both the Commit Session and the Abandon Session actions close the
session, there is afterwards no session context to allow recognition of a repeated
Commit or Abandon. In this case, a repeated Commit elicits a Modbus exception
of 3, Illegal Data, and a repeated Abandon is accepted without error regardless
of its Session ID.
Session Timeout
All transactions of a session must succeed frequently enough so that the duration
between successful transactions does not exceed the session timeout. Each
successful transaction restarts the timeout. If the timeout expires, the session is
automatically abandoned. Other Modbus activity, unrelated to the log-download
session, is not considered, and its only effect upon the timeout would be delays
of session transactions imposed by bandwidth usage.