Date Code 20010518 Appendix G G-7
SEL-2020 Instruction Manual
When referencing coils from most masters, you will need to set the coil number one greater than
the specified coil reference.
Preset Single Register (Function Code 06h)
The SEL-2020 uses this function to allow a Modbus master to write directly to a database
register. Section 9: Database shows which registers are writeable and defines their operation.
If you are accustomed to 4X references with this function code, for 6-digit addressing simply add
400001 to the standard database addresses.
The master request must have the following format:
1 byte Slave Address
1 byte Function Code (06h)
2 bytes Register Address
2 bytes Data
2 bytes CRC-16 for message
A successful SEL-2020 response will be an echo of the request message.
Preset Multiple Registers (Function Code 10h)
This function code works much like code 06h, except that it allows you to write multiple
registers at once, up to 120 per operation. Normally, this function code will only be used in the
USER region. If you are accustomed to 4X references with the function code, for 6-digit
addressing simply add 400001 to the standard database addresses.
The master request must have the following format:
1 byte Slave Address
1 byte Function Code (10h)
2 bytes Starting Address
2 bytes Number of registers to write
1 byte Byte count (should be twice number of registers)
n bytes Byte count bytes of data
2 bytes CRC-16 for Message
A successful response will have the format:
1 byte Slave Address
1 byte Function Code (10h)
2 bytes Starting Address
2 bytes Number of Registers
2 bytes CRC-16 for Message
Report Slave ID (Function Code 11h)
The SEL-2020 identifies the port device type when it receives this request. It also provides
information on how data is being collected from an SEL relay so the specific map to use can be
determined.
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