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10 Appendix
10.1
Modbus Message Format
The Modbus protocol uses serial transmission. One Modbus transaction consists of a Query message
and a Response message. A Modbus-over-ARCNET message consists of:
•
ARCNET Header
•
Modbus Message
The ARC-EVS supports the use of two Modbus functions:
•
Function Read Holding Registers (3H) — which reads the status of the input LEDs on the AEB
•
Function Preset Multiple Registers (10H) — which writes a value to the output LEDs on the AEB
The formatting of these two functions is illustrated below:
Format of the Read Holding Registers Function (3H)
Query Message
System
Code
Split Flag
Sequence Sequence
Number
Number
LSB
MSB
Slave
Address
Function
Number
(3H)
Starting
Address
MSB
Starting
Address
LSB
Slave
Address
Function
Number
(3H)
Byte
Count
Data
MSB
Number of Number of
Points
Points
MSB
LSB
Response Message
System
Code
Split Flag
Sequence Sequence
Number
Number
LSB
MSB
ARCNET Header
Data
LSB
Modbus Message
Input LED Status
Occupies LSB
Data to Output LED
Occupies LSB
Format of the Preset Multiple Registers Function (10H)
Query Message
System
Code
Split
Flag
Sequence Sequence
Function Starting Starting Register Register
Slave
Byte
Number Number
Number Address Address Number Number
Address
Count
(10H)
LSB
MSB
MSB
LSB
MSB
LSB
Data
MSB
Response Message
System
Code
Split Flag
Sequence Sequence
Number
Number
LSB
MSB
Function
Number
(10H)
Slave
Address
ARCNET Header
Byte
Count
Data
MSB
Modbus Message
Contemporary Controls
TD020200-0MA
22
Data
LSB
Data
LSB