Salicru CV10-002-S2 User Manual page 76

Figure 3 is the star connection. Terminal resistor should be connected to the two devices which have the
longest distance. (1# and 15#device)
It is recommended to use shield cables in multiple connection. The basic parameter of the devices, such as
baud rate and digital check bit in RS485 should be the same and there should be no repeated address.
7.2.2 RTU mode
7.2.2.1 RTU communication frame format
If the controller is set to communicate by RTU mode in Modbus network every 8bit byte in the message includes
two 4Bit hex characters. Compared with ACSII mode, this mode can send more data at the same baud rate.
Code system
· 1 start bit
· 7 or 8 digital bit, the minimum valid bit can be sent firstly. Every 8 bit frame includes two hex characters (0...9, A...F)
· 1 even/odd check bit . If there is no checkout, the even/odd check bit is inexistent.
· 1 end bit (with checkout), 2 Bit (no checkout)
Error detection field
· CRC
The data format is illustrated as below:
11-bit character frame (BIT1~BIT8 are the digital bits)
Start bit BIT1
10-bit character frame (BIT1~BIT7 are the digital bits)
Start bit BIT1
In one character frame, the digital bit takes effect. The start bit, check bit and end bit is used to send the digital bit right to the
other device. The digital bit, even/odd checkout and end bit should be set as the same in real application.
The Modbus minimum idle time between frames should be no less than 3.5 bytes. The network device is detecting, even
during the interval time, the network bus. When the first field (the address field) is received, the corresponding device decodes
next transmitting character. When the interval time is at least 3.5 byte, the message ends.
The whole message frame in RTU mode is a continuous transmitting flow. If there is an interval time (more than
1.5 bytes) before the completion of the frame, the receiving device will renew the uncompleted message and
suppose the next byte as the address field of the new message. As such, if the new message follows the
previous one within the interval time of 3.5 bytes, the receiving device will deal with it as the same with the
previous message. If these two phenomena all happen during the transmission, the CRC will generate a fault
message to respond to the sending devices.
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Figure 3 star connection
BIT2 BIT3 BIT4
BIT2 BIT3
BIT5 BIT6
BIT4 BIT5 BIT6
Check
BIT7 BIT8
bit
Check
BIT7
bit
Communication protocol
End bit
End bit