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Chapter 8. MODBUS COMMUNICATIONS FUNCTIONS
• 1-frame end judgment
A message end (1-frame end) is determined when a time period specified for
each transmission speed has passed during which no character is received. It is
considered that 1 frame has ended when the next character is not received before
the time-out time shown below passes.
However, the time-out time has a fluctuation of ±1 ms from the values in the
table below.
Set transmission speed
(bps)
Time-out time
4800
16ms or more
9600
8ms or more
19200
4ms or more
38400
2ms or more
■ Command type
There are two command (send message) types as shown below:
Command
Description
ASCII
RTU (binary)
Read command
"03" (sample)
03H (sample)
Write command
"10" (sample)
10H (sample)
■ Other specifications
• Supporting the MODBUS Class 0
• Abnormal termination codes
Code
01
Description
Command error
02
Address error
03
Data error
• Maximum number of communications data words
Data count
ASCII
RTU
03 (READ)
16
16
16 (WRITE)
16
16
• Other
For the details of MODBUS specifications,
refer to OPEN MODBUS/TCP SPECIFICATION (Release 1.0) by
Modicon Inc.
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