9.13
Date Code 20120126 Instruction Manual SEL-2032 Communications Processor
Protocols
Modbus RTU Protocol
For successful operations, the response message will have the same format as
the request message. For error responses, the message format will be as
follows:
1 byte. Slave Address (echo of received)
1 byte. Exception Function Code (function code with high-bit set)
1 byte. Exception Code (see below)
2 bytes. CRC–16 code for message
Whenever multiple-byte values are sent over Modbus, they are sent most
significant byte first.
The function codes supported by the SEL-2032 are listed in Table 9.4.
Read Coil Status (Function Code 01h)
The SEL-2032 uses function code 01h to read the status of various bits. You
may read up to 1000 bits at once.
The master request must have the following format:
1 byte. Slave Address
1 byte. Function Code (01h)
2 bytes. Starting Bit Address
2 bytes. Number of Bits to Read
2 bytes. CRC–16 for Message
A successful SEL-2032 response will have the following format:
1 byte. Slave Address
1 byte. Function Code (01h)
1 byte. Byte Count
n bytes. Data
2 bytes. CRC–16 for Message
The data response contains 8 bits per data byte, with the LSB of the first byte
corresponding to the addressed bit.
Table 9.5 summarizes the supported bit addresses. Control points are detailed
in Table 9. 6.
Table 9.4 Supported Function Codes
Function Code Description
01h Read Coil Status
02h Read Input Status
03h Read Holding Register
04h Read Input Register
05h Force Single Coil
06h Preset Single Register
10h Preset Multiple Registers
11h Report Slave ID
Courtesy of NationalSwitchgear.com
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