sepam 20 user manual - Schneider Electric

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Modbus communication Disturbance recording
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If the master requests more exchanges than necessary, the exchange number
remains unchanged and the number of usable bytes is forced to 0. To guarantee the
data transfers, it is necessary to allow a response time of about 500 ms between
each reading operation at 2300h.
The first word transmitted is an exchange word. The exchange word comprises two
fields:
b the most significant byte contains the exchange number. It is incremented by 1 by
the Sepam each time a successful transfer takes place. When it reaches the value
FFh, it automatically goes back to zero
b the least significant byte contains the number of usable bytes in the data zone. It
is initialized to zero after energizing and must be different from FFh.
The exchange word may also have the following values:
b xxyy: the number of usable bytes in the data zone yy must be different from FFh
b 0000h: no "read requeste frame" has been formulated yet, as it is the case in
particular, when the Sepam is switched on. The other words are not significant,
b FFFFh: the "request frame" has been processed, but the results in the reply zone
are not yet available.
It is necessary to repeat "reply frame" reading.
The other words are not significant.
The words which follow the exchange word make up the data zone.
Since the configuration and data files are adjacent, a frame may contain the end of
the configuration file and the beginning of the data file of a record.
It is up to the remote monitoring and control system software to reconstruct the files
in accordance with the transmitted number of usable bytes and the size of the files
indicated in the identification zone.
Acknowledging a transfer
To inform the Sepam that a record block that it has just read has been received
correctly, the master must write the number of the last exchange that it has carried
out in the "exchange number" filed and set the "number of usable bytes in the data
zone" of the exchange word to zero.
The Sepam only increments the exchange number if new acquisition bursts are
present.
Rereading the identification zone
To ensure that the record has not been modified, during its transfer by a new record,
the master rereads the contents of the identification zone and ensures that the
recovered record date is still present.
PCRED301005EN - 06/2008