RS485 Appendix - 23
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.
Baud rate (bits/sec) Minimum silent length (ms)
110 350
300 160
600 80
1200 40
2400 18.8
4800 9.5
9600 5.5
19200 3.4
38400 1.6
57600 1.6
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
data direction, and to avoid any computer dependent flow control. Missing to do so may
lead to force the user to work with a slower transmission speed.
An also important feature is the isolation between the RS-232 side and the RS-485 side
to avoid ground loops and to work safely.
Finally, port powered converters should be considered carefully as some computers don't
have sufficient power to feed efficiently the RS-232-RS-485 converter.
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