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2GCS215081A0050_RVT Manual.pdf
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Modicon Modbus Protocol Reference Guide (PI–MBUS–300 Rev. J).
A3 Adequate choice of an RS232-RS485 converter at the computer side
In a RS-485 network, a control of the direction is needed to alternate between
transmission and reception. Two cases may be found:
No flow control:
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The direction control is done in the RS-232 to RS-485 converter. The converter
senses the data direction and release automatically the line to tristate when no
activity is found. The release time is typically one character length (11 bits /
baudrate).
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RTS flow control:
The direction control is done by setting RTS signal (Request To Send) and
releasing RTS after transmission. This is done by software. The drawback
that if the software is busy by another task or if the PC is too slow, the direction
control may be inverted too late, losing received data.
In Modbus RTU framing, messages start with a silent interval of at least 3.5
character times. The table below is related to the RS-485 Modbus Adapter.
It shows the timings of the minimum silent length between the reception of a
query and the transmission of an answer at different Baud rate.
The choice of the RS-232 / RS-485 is then particularly important as a bad management
of the direction of transmission may lead to communication errors or no communication
at all. If RTS is released too early, the query will not arrive to the Modbus slave and no
answer will be initiated. If RTS is released too late, the answer will not be received
completely by the Modbus master.
As a consequence, to ensure higher throughput and reliable communication, the
recommendation is done to choose an RS-232 / RS-485 able to sense and manage the
22 – Appendices ç RS-485
Baud rate (bits/sec)
110
300
600
1200
2400
4800
9600
19200
38400
57600
Minimum silent length (ms)
350
160
80
40
18.8
9.5
5.5
3.4
1.6
1.6
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