E.17
Date Code 20170927 Instruction Manual SEL-751 Relay
Modbus RTU Communications
Communications Protocol
Reading Parameter
Information and Value
Using Modbus
Through use of Modbus commands, you can read the present value of a
parameter as well as parameter name, units, low limit, high limit, scale, and
even the enumeration string (if the parameter is an enumeration type). This
means that you can use a general user interface to retrieve and display specific
parameter details from the relay. Use the 60h, 61h, and 62h commands to
retrieve parameter information, and use the 03 command to retrieve values.
Controlling Output
Contacts Using
Modbus
The SEL-751 includes registers for controlling some of the outputs. See
LOGIC COMMAND (2000h), RESET COMMAND (2001h), and registers in
the Reset Settings region for the control features supported by the relay. Use
Modbus function codes 06h or 10h to write appropriate flags. Remember that
when writing to the Logic command register with output contacts, it is not a
bit operation. You must write all the bits in that register together to reflect the
state you want for each of the outputs.
User-Defined Modbus
Data Region and
SET M Command
The SEL-751 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. You can also define the user map by
writing to user map registers MOD_001 to MOD_125.
To use the user-defined data region, perform the following steps.
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) by using the
labels in Table E.33.
Note that you can also use Modbus protocol to perform this
step. 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 quantities
you want from addresses 126 through 250 (user map values).
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