Unitronics UMI-B1 UL Series Operation Manual page 114

UMI-B1 UL Series Inverter
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)
BIT
Start bit
1
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.
The standard structure of RTU frame:
START
ADDR
BIT BIT BIT
2 3 4
BIT
BIT3 BIT4
2
T1-T2-T3-T4 (transmission time of 3.5 bytes)
Communication address: 0–247 (decimal system) (0 is the
broadcast address)
BIT BIT BIT
5 6 7
BIT5 BIT6
112
Communication protocol
BIT Check
8 bit
Check
BIT7
bit
End
bit
End
bit