YOKOGAWA UTAdvanced Series, User Manual

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3-2
IM 05P07A01-01EN
Overview of Modbus Communication Connection
RS-232C
PC
RS-485
SCADA software
(commercially available)
ML2
RS-485/
RS-232C
converter
Up to 31 connected slaves with a maximum length of 1200 m
Model: ML2 of YOKOGAWA is recommended.
There are two transfer modes in Modbus communication, the ASCII mode and RTU
mode (binary method).
When multiple controllers are connected on a single network, all of the controllers must
be set to the same transfer mode.
The following table compares the RTU and ASCII modes.
Comparison of RTU and ASCII Modes
Item ASCII Mode RTU Mode
Number of data bits 7 bits (ASCII) 8 bits (binary)
Detection of message start Text
: (colon)
Time
Data receive after the silent
interval of 3.5 characters or more
Detection of message end Text
CR+LF
Time
Silent interval of 3.5 characters or
more after data receive
Message length (*1) 2N+1 N
Inter-character timer timeout 1 second None
Error detection Longitudinal redundancy
check: LRC
Cyclic redundancy check:CRC-16
*1: The message length in the ASCII mode becomes “ 2N+1 ” when the message length in the
RTU mode is assumed to be “ N. ”
Detection of Message Frame Termination in RTU Mode
Frame 1 Frame 2
Time
3.5 chars or more 3.5 chars or more
Note
When the write data to D registers / I relays is outside the valid range, the response is returned
as “normal.”
However, the data cannot be written to D registers / I relays.
For the 2-wire connection of the UTAdvanced series with DAQSTATION series, set the
"Command wait time" of the DAQSTATION to 10 ms or more.
In case of the 2-wire connection with the MW100 Data acquisition unit via Modbus
master function, set the "Gap between messages" of MW100 to 10 ms or more.
3.1 Modbus Communication