Appendix A. Modbus Reference - Banner A-GAGE EZ-ARRAY Instruction Manual

Appendix

Appendix A. Modbus Reference

A.1 Modbus Specifications and Message Formats
For the latest Modbus protocol and specifications, please visit
http://www.modbus.org
The EZ-ARRAY is compliant with Modbus v1.1a. EZ-ARRAY
utilizes the RTU transmission Mode. The RTU message frame is
depicted below.
Table A–1. RTU Message Frame
Slave Function
Address Code
1 byte 1 byte
The slave address can be assigned a value in the range 1-247.
Three function codes are supported:
(0x03) Read Holding Registers
(0x04) Read Input Registers
(0x10) Write Multiple Holding Registers
The data in the registers is transmitted in big endian order
(HIGH byte, LOW byte).
A.1.1 Read Holding Registers (0x03)
This function code is used to read the contents of a contiguous
block of EZ-ARRAY holding registers. The command specifies
the starting address and the number of registers. The EZ-
ARRAY employs a direct addressing scheme. For example,
the holding register at address 40001 is accessed by reading
address 40001 (0x9C41) directly (i.e. the starting address is
not an offset). The register data in the response message are
packed as two bytes per register. For each register, the data is
returned in a big endian order (HIGH byte, LOW byte).
Request
Function Code
1 byte
Starting Address 2 bytes
Quantity of 2 bytes
Holding Registers
Response
Function Code 1 byte
Byte Count
1 byte
Holding Registers N X 2 Bytes
* "N" is the number of holding registers
34
P/N 130426 Rev. D
Cyclic
Data Redundancy
Check (CRC)
0 up to 252
2 bytes (CRC
bytes(s) (High
Low, CRC Hi)
Byte, Low Byte)
0x03
0x0000 to 0xFFFF
0x0001 to 0x007D
0x03
2 X N*
Error
Error Code 1 byte
Exception Code 1 byte
Example A–1. Reading Holding Registers
Request
Field Name (Hex)
Slave Address 41
Function 03
Starting Address 9C
(HIGH Byte)
Starting Address 41
(LOW Byte)
Quantity of Registers 00
(HIGH Byte)
Quantity of Registers 03
(LOW Byte)
CRC (LOW Byte) 75
CRC (HIGH Byte) 4F
The contents of addresses 40001, 40002, and 40003 are the
two-byte values 0x022B (555 decimal), 0x0000 (0 decimal), and
0x0064 (100 decimal), respectively.
A.1.2 Read Input Registers (0x04)
This function code is used to read from 1 to 125 contiguous
EZ-ARRAY input registers. The command specifies the starting
address and the number of registers. The EZ-ARRAY employs
a direct addressing scheme. For example, the input register at
address 30001 is accessed by reading address 30001 (0x7531)
directly (i.e. the starting address is not an offset). The register
data in the response message are packed as two bytes per
register. For each register, the data is returned in big endian
order (HIGH byte, LOW byte).
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A-GAGE EZ-ARRAY
Instruction Manual
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Response
Field Name (Hex)
Slave Address 41
Function 03
Byte Count 06
Register 40001 02
(HIGH Byte)
Register 40001 2B
(LOW Byte)
Register 40002 00
(HIGH Byte)
Register 40002 00
(LOW Byte)
Register 40003 00
(HIGH Byte)
Register 40003 64
(LOW Byte)
CRC (LOW Byte) 34
CRC (HIGH Byte) B9
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