E.5
Date Code 20170927 Instruction Manual SEL-751 Relay
Modbus RTU Communications
Communications Protocol
To build the response, the device calculates the number of bytes necessary to
contain the number of bits requested. If the number of bits requested is not
evenly divisible by eight, the device adds one more byte to maintain the
balance of bits, padded by zeroed to make an even byte.
In each row, the input numbers are assigned from the right-most input to the
left-most input (i.e., Input 1 is TLED_06 and Input 8 is ENABLED). Input
addresses start at 0000 (i.e., Input 1 is located at Input Address 0000).
Table E.7 includes the coil address in decimal and lists all possible inputs
(Relay Word bits) available in the device. Note that the command depends on
the device hardware configuration; the device responds only to installed cards.
2 bytes Number of bits to read
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (02h)
1 byte Bytes of data (n)
n bytes Data
2 bytes CRC-16
Table E.7 02h SEL-751 Inputs
Coil Address (Decimal) Function Code Supported Coil Description
a
a
The input numbers are assigned from the right-most input to the left-most input in the Relay
row as show in the following example.
Address 7 = ENABLED
Address 6 = TRIP
Address 5 = T01_LED
Address 4 = T02_LED
Address 3 = T03_LED
Address 2 = T04_LED
Address 1 = T05_LED
Address 0 = T06_LED
0–7 2 Relay Element Status Row 0
8–15 2 Relay Element Status Row 1
16–23 2 Relay Element Status Row 2
•
•
•
•
•
•
•
•
•
1312–1319 2 Relay Element Status Row 164
1320–1327 2 Relay Element Status Row 165
1328–1335 2 Relay Element Status Row 166
Table E.6 02h Read Input Status Command (Sheet 2 of 2)
Bytes Field
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