Schweitzer Engineering Laboratories SEL-701 - 01 H Read Coil Status Command

Date Code 20010719 701 Motor Protection Relay
Modbus
®
RTU Communications Protocol
01h Read Coil Status Command
C.5
01h Read Coil Status Command
Use function code 01h to read the On/Off status of selected bits (coils). 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.4 01h Read Coil Sta tus 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.4 01h Read Coil Status Command
Bytes Field
The Master Request Must Have the Following Format
1 byte Slave address
1 byte Function code (01h)
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 (01h)
1 byte Byte count
n bytes Byte count bytes of data
2 bytes CRC-16 for message