Date Code 20010518 Settings 6-19
SEL-2020 Instruction Manual
number of items (NUMn) to 4, the following message is parsed as shown below (all data shown
as hexadecimal character codes):
Message: 019902109903249915FF9934C80B
Parsed result: 011310992415FF34
Notice that the NUMn setting applies to the number of “parsed” items, not to the number of
received items. This parsing method is useful when binary data are being received while
XON/XOFF flow control is enabled. When communicating with another SEL-2020, the \Rx.../
and \Ry.../ strings can be used in the downstream SEL-2020 to encode the data before sending it
to the upstream 2020. See Section 8: Message Strings for more detailed information.
Parsing Delays
When any of the above “generic” parsing methods is used (Parse = 1,2,3,4,5), the SEL-2020 uses
the NUMx setting to determine when to stop collecting data items. If the SEL-2020 has not
received the specified number of items, it will continue to wait for them until a predetermined
amount of time has passed without receipt of a new item. This time delay is 5 seconds for ports
with DEVICE set to Other-IED and 15 seconds for ports with DEVICE set to SEL-IED. Once
this amount of time passes, the SEL-2020 takes the data items that it has received and continues
to the next step in the parsing process, either performing checksum validation or simply storing
the data to the database.
If the SEL-2020 receives the number of items specified by the NUMx setting, the next task is
determined by the DELAYx setting. If the DELAYx setting is set to “ON”, then the SEL-2020
will execute the same type of delay as described above, ignoring any received items until no
more items are received for a fixed time interval. It then moves on to the next step in the parsing
process. If the DELAYx setting is set to “OFF’, then the SEL-2020 will immediately move on to
the next step in the parsing process. Any characters received beyond the number of expected
data items may end up in the Unsolicited Message Buffer or may even be captured by a
subsequent data collection process. Setting the DELAYx setting to “ON” helps to ensure that
excess characters in a device response will not be treated as part of a subsequent request-
response sequence. This introduces time delays in the parsing process, preventing rapid
successive data collections. When it is known that the responding device will send a fixed
number of items without any excess trailing characters, setting DELAYx to “OFF” may be
preferable because this enables the parsing process to complete quickly, allowing for rapid
successive data collections.
Checksum Validation
If you choose a parse type of Character String (PARSEn=3), Integer String (PARSEn=4), or
Integer String with XON/XOFF encoding (PARSEn=5), you can set the SEL-2020 to perform
checksum validation on the parsed response. The CHECKn setting specifies the type of
checksum being used (CRC-16, 8-bit checksum, or 16-bit checksum) and the format of the
checksum (ASCII hexadecimal or binary). The ORDERn setting specifies the byte ordering of
the checksum for CRC-16 and 16-bit checksums. The STARTn, STOPn, and CHKPOSn settings
specify the locations of the data to be validated and the checksum in the received data stream.
Three methods are available for specifying these position settings: 1) specify a byte index in the
received data stream, where byte index 1 is the first position, 2) specify a character or character
code, where a trailing ‘i’ can be appended to indicate that the character itself is included, 3)
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