E.20
SEL-751A Relay
Instruction Manual
Date Code 20100129
Modbus RTU Communications
Communications Protocol
setting interdependency rule is violated, the settings are not saved. The relay
will set the Config Fault bit in the TRIP STATUS HI register to indicate that
the save settings operation has failed. The relay will also set ERROR
REGISTER (4016h) flags to indicate the type of error.
Parameters such as date and time can be changed with the appropriate
registers by using Modbus Function Code 06h or 10h.
Controlling Output
Contacts Using
Modbus
The SEL-751A 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. All the bits in that register need to be written together to reflect
the state you want for each of the outputs.
User-Defined Modbus
Data Region and
SET M Command
The SEL-751A 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 (up to 125). Arrange the
quantities in any order that is convenient for you to use.
Step 2. Refer to
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
.
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).
Summary of Contents for 751A
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