Date Code 20010518 Appendix G G-5
SEL-2020 Instruction Manual
The master request must have the following format:
1 byte Slave Address
1 byte Function code (03h)
2 bytes Starting database address
2 bytes Number of registers to read
2 bytes CRC-16 for message
A successful SEL-2020 response will have the format:
1 byte Slave Address
1 byte Function code (03h)
1 byte Byte count (should be twice number of registers read)
n bytes Byte Count Bytes of Data
2 bytes CRC-16 for Message
Read Input Register (Function Code 04h)
The SEL-2020 uses function code 04h for reading from a Modbus specific map. This map has
various kinds of data at specific addresses, independent of user settings. You must merely
collect the data in some region, and it will be visible in this register map. In this map, you can
also select whether to use the standard-style, which includes floating-point data, or an integer-
only map, based on a setting. You may read a maximum of 125 registers at once with this
function code.
The master request must have the following format:
1 byte Slave Address
1 byte Function Code (04h)
2 bytes Starting database address
2 bytes Number of registers to read
2 bytes CRC-16 for message
A successful SEL-2020 response will have the format:
1 byte Slave Address
1 byte Function Code (04h)
1 byte Byte Count (should be twice number of registers read)
n bytes Byte count bytes of data
2 bytes CRC-16 for message
Tables G.1 through G.10 contain register maps for meter data, demand meter data, history data,
target data, and breaker data from various SEL relays for 04h accesses. You can use these maps
to identify the registers that contain the data you want to collect through the Modbus port. These
maps contain the same data available in database regions, but it is arranged differently. Most
masters use 3x references when accessing input registers. To use this reference method with 5-
digit addressing, simply add 30001 to the address in the tables. Floating-point items are
transferred most-significant word first. Make sure your master can accept floating-point data in
this format before using it.
User region registers can be accessed using 5-digit addressing starting at 32401 and following.
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