G-4 Appendix G Date Code 20010518
SEL-2020 Instruction Manual
A successful SEL-2020 response will have the following format:
1 byte Slave Address
1 byte Function Code (01h)
1 byte Byte Count
n bytes Data
2 bytes CRC-16 for Message
The data response contains 8 bits per data byte, with the LSB of the first byte corresponding to
the addressed bit.
The following table lists the supported bit addresses:
Bit Addresses
Corresponding Database Register
1000h - 100Fh Global Status Register
1010h - 101Fh Global Configuration Register
1020h - 1057h 7 Global Element Registers (low-byte only)
1058h - 105Fh Reserved - Always 0
1060h - 106Fh Local Status Register
1070h - 10FFh 18 Local Element Registers (low-byte only)
1100h - 15FFh Target Region Targets (low-bytes only)
1600h- User Region Registers
In all cases, bit numbering starts with the LSB of each register. See Section 9: Database for a
description of these registers. To access relay target data, you must set a region to collect the
target data. Then, using the command MAP n TARGET BL, you can determine how many bytes
of target data exist and what each bit is. The first target element is accessible at 1100h.
When referencing the data from most masters, you will need to set the coil number one greater
than the listed bit address.
Read Input Status (Function Code 02h)
Function code 02h is used in a manner identical to function code 01h, as discussed above. Most
masters use 1X references with this function code. To find the 1X reference with 5-digit
addressing, add 10001 to the bit address specified above.
Read Holding Register (Function Code 03h)
The SEL-2020 uses function code 03h for reading from the database directly. Chapter 9
discusses this database. Use the MAP command to determine the details of the register maps
based on your settings. You may read a maximum of 125 registers at once with this function
code. Most masters use 4X references with this function code. To find the 4X reference with 5-
digit addressing, add 40001 to the database addresses.
From a Modbus Master, reading registers in the database with addresses above 9999 (270Fh),
requires 6-digit addressing to avoid corrupting the type identifier digit 4. For example, the first
register in the user region is at address F800h which translates to 463489 in 6-digit addressing.
To read the user region with 5-digit addressing, access registers as 3X using function code 04.
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