E Q U O B O X R T U - M o d B u s
–
U s e r G u i d e R e v . 2 . 2
S I N A P S I S . r . l . |
V i a d e l l e Q u e r c e 1 1 / 1 3 - 0 6 0 8 3 B A S T I A U M B R A ( P G )
–
I t a l y
T . + 3 9
0 7 5 8 0 1 1 6 0 4
- F . + 3 9
0 7 5 8 0 1 4 6 0 2 |
w w w . s i n a p s i t e c h . i t
i n f o @ s i n a p s i t e c h . i t
6
4.
GENERAL INFORMATION ABOUT ModBus ARCHITECTURE
4.1
System overview
ModBus is a serial communication protocol created by Modicon in 1979 . It became standard in industrial
communication , and is currently one of the most popular connection protocols in the world between industrial
electronic devices. The main reasons for such a high usage compared to other ModBus communication
protocols are:
•
It is a protocol openly published and royalty-free
•
Move raw bits and words without placing many restrictions on producers
The protocol defines the format and the communication mode between a "master" who manages the system
and one or more "slav
es”
that respond to the master. The Protocol defines how the master and the slaves and
disrupt communication, such as transmitter and receiver must be identified, such as messages need to be
exchanged and how the errors detected.
Only the master can start a transaction that can have the question/answer format directly to a single slave or
broadcast when the message is sent to all devices on the line that give no response. A transaction consists of
a single structure question/single answer or a single broadcast message/no answer. Some features of the
protocol are:
•
Baud rate
•
Parity
•
Number of stop bits
•
and size RTU (binary)
As mentioned above the transactions always involve the ModBus master, which operates the line, and one
slave at a time (except in the case of broadcast messages). To identify the recipient of the message is sent as
the first character a byte containing the numeric address of the selected slave device. Each slave will then be
assigned a different number of address that uniquely identifies it. Admissible address are from 1 to 254 , while
the address 0, which can’
t be assigned to a slave, place on top of the message sent by the master indicates
that it is "broadcast", that is directed to all slaves simultaneously.
Below is showed a standard architecture for the physical connection of n ModBus devices.
Figure 2 - System architecture
In the RS485 signal cable laying be careful to
•
Pass the cable away from the power cables to avoid interference EMI
