E.19
Date Code 20150130 Instruction Manual SEL-787 Relay
Modbus RTU Communications
Communications Protocol
User-Defined Modbus
Data Region and
SET M Command
The SEL-787 Modbus Register Map defines an area of 125 contiguous
addresses whose contents are defined by 125 user-settable addresses. This
feature allows you to take 125 discrete values from anywhere in the Modbus
Register Map and place them in contiguous registers that you can then read in
a single command. SEL ASCII command SET M provides a convenient
method to define the user map addresses. The user map can also be defined by
writing to user map registers MOD_001 to MOD_125.
To use the user-defined data region, follow the steps listed below.
Step 1. Define the list of desired quantities (as many as 125). Arrange
the quantities in any order that is convenient for you to use.
Step 2. Refer to Table E.33 for a list of the Modbus label for each
quantity.
Step 3. Execute SET M command from the command line to map user
registers 001 to 125 (MOD_001 to MOD_125) using the labels
in Table E.33.
Note that this step can also be performed using Modbus
protocol. Use Modbus Function Code 06h to write to registers
MOD_001 through MOD_125.
Step 4. Use Modbus function code 03h or 04h to read the desired
quantities from addresses 126 through 250 (user map values).
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