A3 Adequate Choice Of An Rs232-Rs485 Converter At The Computer Side - Hitachi Abb Rvt Installation Und Betriebsanleitung

Tri-State
Tx
Unsigned Integer
Word
Xon/Xoff
A2 References
– 9AKK106713A9758_RVT communication with Modbus RTU, Modbus TCP and PQ Link protocols
– 9AKK107045A9645_RVT Manual.pdf
– 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:
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).
– 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 is 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 re-
lated 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.
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
STATUS
Approved
The ability of a communications transmitter to turn its circuitry off, reducing the load on the
network
see Transmit
Positive whole number value represented by 16 bits (0 to 65535)
A group of 16 bits
Software implementation of data flow control
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DOCUMENT ID
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RS 48 5 A DAPTE R
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