Sel 751A, Instruction Manual

Page: 467 / 640

E.5
Date Code 20100129 Instruction Manual SEL-751A Relay
Modbus RTU Communications
Communications Protocol
To build the response, the device calculates the number of bytes required 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 zeroes 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.
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-751A Inputs (Sheet 1 of 4)
Coil Address
(Decimal) Function Code
Supported
Coil Description
a
0–7 2 Relay Element Status Row 0
8–15 2 Relay Element Status Row 1
16–23 2 Relay Element Status Row 2
24–31 2 Relay Element Status Row 3
32–39 2 Relay Element Status Row 4
40–47 2 Relay Element Status Row 5
48–55 2 Relay Element Status Row 6
56–63 2 Relay Element Status Row 7
64–71 2 Relay Element Status Row 8
72–79 2 Relay Element Status Row 9
80–87 2 Relay Element Status Row 10
88–95 2 Relay Element Status Row 11
96–103 2 Relay Element Status Row 12
104–111 2 Relay Element Status Row 13
112–119 2 Relay Element Status Row 14
120–127 2 Relay Element Status Row 15
128–135 2 Relay Element Status Row 16
136–143 2 Relay Element Status Row 17
144–151 2 Relay Element Status Row 18
152–159 2 Relay Element Status Row 19
160–167 2 Relay Element Status Row 20
168–175 2 Relay Element Status Row 21
Table E.6 02h Read Input Status Command (Sheet 2 of 2)
Bytes Field