Chapter 9 Communications
9.3 MODBUS Communication Protocol
When using RS-232/485 serial communication interface, each ASDA-A2 series AC servo drive
has a pre-assigned communication address specified by parameter “P3-00”. The computer
then controls each AC servo drive according to its communication address. ASDA-A2 series
AC servo drives can be set up to communicate on a MODBUS network using on of the
following modes: ASCII (American Standard Code for Information Interchange) or RTU (Remote
Terminal Unit). Users can select the desired mode along with the serial port communication
protocol in parameter “P3-02”.
Code Description
ASCII Mode:
When AC servo drives are set up to communicate on a MODBUS network using ASCII
(American Standard Code for Information Interchange) mode, each 8–bit data in a message is
sent as two ASCII characters between the master and the slave. For example, a 1-byte data:
64 Hex, shown as ‘64’ in ASCII, consists of ‘6’ (36Hex) and ‘4’ (34Hex).
ASCII Characters:
Character ‘0’ ‘1’ ‘2’ ‘3’ ‘4’ ‘5’ ‘6’ ‘7’
ASCII code 30H 31H 32H 33H 34H 35H 36H 37H
Character ‘8’ ‘9’ ‘A’ ‘B’ ‘C’ ‘D’ ‘E’ ‘F’
ASCII code 38H 39H 41H 42H 43H 44H 45H 46H
RTU Mode:
When AC servo drives are set up to communicate on a MODBUS network using RTU (Remote
Terminal Unit) mode, each 8-bit data is the combination of two 4-bit hexadecimal characters.
For example, a 1-byte data: 64 Hex. Comparing to ASCII mode, the transmission speed of RTU
mode is better.
Data Format
10-bit character frame (For 7-bit character)
7N2
Start
bit
01
2
3456
Stop
bit
7-data bits
10-bits character frame
Stop
bit
7E1
Even
parity
Start
bit
01
2
3456
Stop
bit
7-data bits
10-bits character frame
Revision January 2011 9-5
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