SRV-63 series AC servo drives Communication
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salve needs not to do so.
8.2.3 Communication frame structure
Modbus protocol supports RTU mode only. The user can set serial communication parameters, such
as, the baud rate and the checkout means.
8.2.3.1 RTU mode
Each 8bit bytes in the message frame contains two 4bit hex characters.
Table 8-1: The message frame in RTU mode
The start bit
Device address Command code
Data
LRC checkout
The tailed
T1-T2-T3-T4
8Bit
8Bit
n 8Bit(s)
16Bit
T1-T2-T3-T4
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 hap pen during the
transmission, the CRC will generate a fault message to respond to the sending devices.
8.2.4 Command code and the communication data instructions
8.2.4.1 Command code: 03H
Function: read N words (can read no more than 16 words continuously).
For example, the servo drive with the salve address of 01H, if its starting address is 03F2H, read 2
words continuously, and then the structure of the frame is:
Table 8-2 The master device request command
START
T1-T2-T3-T4 (transmission time of 3.5 bytes)
ADDR
01H
CMD
03H
High bit of start address
03H
Low bit of start address
F2H
High bit of data number
00H
Low bit of data number
02H
Low bit of CRC CHK
65H
High bit of CRC CHK
BCH
END
T1-T2-T3-T4 (transmission time of 3.5 bytes)