Communication Frame Structure; Command Codes - INVT DA180-S2R8SG0 Operation Manual

DA180 series basic AC servo drive
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.
Start bit
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:
Read MSB in start address
Read LSB in start address
MSB of data count (in word)
LSB of data count (in word)
Table 8-1 RTU message frame
Device
Command
address
8Bit 8Bit
Table 8-2 Master-node command message
START
ADDR
CMD
Data
n * 8 bits
T1-T2-T3-T4 (3.5-byte transmission time)
-244-
Communication
CRC Stop bit
16Bit T1-T2-T3-T4
01H
03H
03H
F2H
00H
02H