7.22
SEL-2032 Communications Processor Instruction Manual Date Code 20120126
Settings
Automatic Message (SET A) Settings
Suppose the string below is sent to the SEL-2032 and you wish to verify that
there are no transmission errors. Assume that the checksum is calculated on
the data within the quotes. In this case the checksum is a 16-bit checksum in
ASCII hexadecimal format with the high byte first. There are many different
ways that you could specify the locations of the data and checksum. Four
examples are given.
received data: “This is data”, 044E
settings:
CHECK = 16A,
ORDER = H,
method 1: START = 2, STOP = 13, CHKPOS = 16
method 2: START = E18, STOP = E6, CHKPOS = E4
method 3: START = “, STOP = ”, CHKPOS = ‘,’
method 4: START = 2, STOP = ”, CHKPOS = E4
The method you choose for each of the position settings depends on the
format of the received data. These position settings apply only to the data to be
stored. This means that the NUMn setting must be large enough to contain all
of the data and the checksum. Otherwise, the checksum verification will
consistently fail. The position settings must be sequential in the received data:
the STARTn position must be on or before the STOPn position and the STOPn
position must precede the CHKPOSn position.
The ACKn and NACKn settings allow you to set strings to be sent following
successful or failed checksum validation, respectively. The content of these
strings is limited to characters and character codes. None of the special
SEL-2032 strings are allowed.
When the SEL-2032 is set to do checksum verification, data are only stored to
the database when the checksum verification is successful. If the checksum
verification fails, the SEL-2032 rerequests the data by sending the NACKn
string if one is set. If no NACKn string is set, the MESGn string is sent again.
The SEL-2032 then parses the data and attempts checksum verification again.
If this verification fails again, one final attempt (for a total of three) is made. If
the NACKn string was sent previously and resulted in no response at all, the
final rerequest is made using the MESGn string. If the checksum verification
is successful, the data are stored to the database (including the checksum) and
the ACKn string is sent to the connected device.
Courtesy of NationalSwitchgear.com
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