AS64 series AC servo drive Communication
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devices with the same communication protocol. The master node can not only communicate with one
slave node but also send broadcast message to all. For a separate "query/command" from the master
node, the slave node must return a response message; for a broadcast message from the master
node, slave nodes do not need to respond.
8.2.3 Communication frame structure
Modbus supports the RTU transmission mode only. You can set the serial-port communication
parameters (including the baud rate and check method). In an RTU message frame, each 8-bit byte
consists of two 4-bit hexadecimal characters.
Table 8-1 RTU message frame
Start bit
Device
address
Command
Data
CRC
Stop bit
T1-T2-T3-T4
8Bit
8Bit
n * 8 bits
16Bit
T1-T2-T3-T4
In this mode, each message must be preceded by a time gap with a minimum length of 3.5 characters.
During the transmission, the network device continuously detects the network bus even within the time
gap. When the first domain (or address domain) is received, the corresponding device decodes the
subsequent transmission characters. The message ends only when there is a time gap with a minimum
length of 3.5 characters.
An entire RTU message frame must be transmitted as a continuous flow. If a receiver detects a time
gap with a minimum length of 1.5 characters before the frame ends, the receiver refreshes the
incomplete message and assumes that the next byte is the address domain of a new message.
Similarly, if a new message follows the previous message within the time gap with a length of less than
3.5 characters, the receivers considers the new message as the continuity to the previous message.
If either of the case occurs, a CRC error message is generated and sent back to the sender.
8.2.4 Command codes
8.2.4.1 Command code 03H
Function: to read
n
words (a maximum of 16 words can be read continuously)
Example: If the command is to continuously read two words from the memory start address 03F2H
of the servo drive with the slave-node address 01H, the command frame structure is as follows:
Table 8-2 Master-node command message
START
T1-T2-T3-T4 (3.5-byte transmission time)
ADDR
01H
CMD
03H
Read MSB in start address
03H
Read LSB in start address
F2H
MSB of data count (in word)
00H
LSB of data count (in word)
02H
