Date Code 20010518 Settings 6-21
SEL-2020 Instruction Manual
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-2020 strings are allowed.
When the SEL-2020 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-2020 will re-
request the data by sending the NACKn string if one is set. If no NACKn string is set, the
MESGn string will be sent again. The SEL-2020 will then parse the data and attempt checksum
verification again. If this verification fails again, one final attempt (for a total of three) will be
made. If the NACKn string was sent previously and resulted in no response at all, the final re-
request will be 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.
SET A Prompts and Settings
When you issue the SET A command for a specific port, the SEL-2020 prompts you for
responses based on the type of device connected to that port. There are four device types as
outlined below.
• SEL-IED device port (Table 6.7)
• Other IED device port (Table 6.8)
• Printer device port (Table 6.9)
• Master device port (Table 6.10)
Detailed descriptions of the SET A Auto-Message Settings are described in Table 6.11.
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