E.25
Date Code 20170927 Instruction Manual SEL-751 Relay
Modbus RTU Communications
Communications Protocol
Reading History Data
Using Modbus
Through use of the Modbus Register Map (Table E.34), you can download a
complete history of the last 50 events via Modbus. The history contains the
date and time stamp, type of event that triggered the report, currents, and
voltages at the time of the event. Please refer to the Historical Data section in
the map.
To use Modbus to download history data, write the event number (1–50) to the
EVENT LOG SEL register at address 1692 (when event number zero is
written by accident for the first time, the relay treats selected event number as
one). Then read the history of the specific event number you requested from
the registers shown in the Historical Data section of the Modbus Register Map
(Tabl e E. 34 ). After a power cycle, the history data registers show the history
data corresponding to the latest event. This information updates dynamically;
as whenever there is a new event, the history data registers update
automatically with new event data. If specific event number data have been
retrieved using a write to the EVENT LOG SEL register, the event data
registers stay frozen with that specific event history. These registers return to
the free running latest event history data mode when a zero is written to the
event selection register from a prior nonzero selection.
Modbus Register Map
Table E.34 lists the data available in the Modbus interface and their
description, range, and scaling information. The table also shows the
parameter number for access through use of the DeviceNet interface. The
DeviceNet parameter number is obtained by adding 100 to the Modbus
register address.
NOTE: Certain Modbus quantities
are reported as 32-bit numbers;
however, Modbus registers are only
16-bit. This results in displaying the
Modbus quantities in a LOW and HIGH
register. To determine the 32-bit
number concatenate the LOW register
to the end of the HIGH register. For
example, if the HIGH register value is
0x5ADC and the LOW register value is
0xF43B, the resulting 32-bit value is
0x5ADCF43B.
Table E.34 Modbus Register Map
a
(Sheet 1 of 38)
Modbus Register
Address
b
Name/Enums Units Min Max Default
Scale
Fac to r
DeviceNet
Parameter
Numbers
0(R) Reserved
c
100
User Map Register
1 (R/W) USER REG #1 280 1935 320 1 101
•
•
•
125 (R/W) USER REG #125 280 1935 1920 1 225
User Map Register Values
126 (R) USER REG#1 VAL 0 65535 0 1 226
•
•
•
250 (R) USER REG#125 VAL 0 65535 0 1 350
Reserved Area 1
251–260 (R) Reserved
c
351–360
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