Schweitzer Engineering Laboratories SEL-701, Instruction Manual

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701 Motor Protection Relay
Date Code 20010719
Modbus
®
RTU Communications Protocol
02h Read Input Status Command
C.6
02h Read Input Status Command
Use function code 02h to read the On/Off status of selected bits. You may read the
status of up to 2000 bits per query. Note that the relay addresses inputs from 0. The
relay returns 8 bits per byte, most significant bit first, with zeroes padded into
incomplete bytes.
Table C.5 02h Read Input Status Command
To build the response, the relay calculates the number of bytes required to contain
the number of bits requested. If the number of bits requested is not evenly divisible by
8, the relay adds one more byte to contain the balance of bits, padded by zeroes to
make an even byte.
Table C.5 02h Read Input Status Command
Bytes Field
The Master Request Must Have the Following Format
1 byte Slave address
1 byte Function code (02h)
2 bytes Address of the first bit
2 bytes Number bits to read
2 bytes CRC-16 for message
A Successful SEL-701 Relay Response Will Have the Following Format
1 byte Slave address
1 byte Function code (02h)
1 byte Byte count
n bytes Byte count bytes of data
2 bytes CRC-16 for message