AI895 Section 3 Run-time Operation
98 3BSE027630-600 B
– Instrument time-out: when the maximum amount of time that an I/O
module will wait before deciding that the instrument has failed is elapsed.
The value is fixed to 33 character times (305 ms)
– Outgoing communication error: when bit 7 is set in the reply status first
byte
– Incoming communication error:
- byte count error (received data byte number differs from byte count field)
- command number error (received command number differs from
transmitted one)
- received address error
- check sum error
- character error (parity, overrun, framing error)
Default number of retries is 2.
• Scan enabled
The scanning function is implemented to monitor the presence of field
instruments.
When the scanning is enabled or if diagnosis is read all instruments of the
instruments list are scanned (a different channel every 5 s).
If an instrument does not reply the DISAPPEARED and SEARCHING bits
related to that instrument are set. Then the module begins to scan that
instrument with HART command 0.
If there is a reply to command 0, the module verifies if it comes from the
original instrument:
– if it does, the APPEARED bit is set, the DISAPPEARED and
SEARCHING bits are cleared and the module restarts the normal
scanning activity;
– if it does not, the MISMATCHED bit is set, the DISAPPEARED bit is
cleared and the module keeps on searching the instrument with
command 0.
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