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CP347 – User Guide, Rev. 1.3
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Handshake Control in Half Duplex RS485 Mode 3.1.3.
In the two-wire half duplex RS485 mode, a handshake mechanism is necessary to prevent sending and receiving data
at the same time over the same RS485 cabling.
T
he handshake mechanism manages the driver enable (DE) and receiver enable (RE) signals of the
RS422/RS485 transceivers. The CPLD logic receives the RTS# or DTR# signals from the serial controller
and generates the DE and RE signals as outputs for the transceivers. The auto RS485 feature enables
either the RTS# or DTS# outputs signals to control the RS485 enable input.
The CP347 RTS# or DTR # control signal to the CPLD are configurable through resistor assembly. For
more information on the required resistor configuration, see table 9 below where ’Open’ means a resistor
is not assembled and ‘Set’ means an assembled 0
Ω
resistor.
Table 9: RS485 Transceiver Control (RTS# and DTS#) Resistor Configuration
Channel 0 Channel 1 Channel 2 Channel 3
R32 R33 R45 R44 R46 R47 R38 R37
RTS# Signal Controls
DE/RE (default) Open Set Open Set Open Set Open Set
DTR# Signal Controls
DE/RE# Set Open Set Open Set Open Set Open
Depending on whether the RS485 mode is selected with or without local echo the receiver RE signal is always ‘Low’
(with local echo) or the RE signal is always ‘High’ (without local echo).
With local echo, the internal receiver is always activated and the RS422/RS485 transceivers receive transmitted
signals instantaneously.
Without local echo, to receive data the receiver is activated and the transmitter is deactivated, and to send data the
transmitter is activated and the receiver is deactivated.
The hardware implementation works with low active DTR#/RTS# handshaking
3.2. Software Configuration
Configure as required using the drivers for Windows®, Linux® or VxWorks®. For more detailed driver information, refer
to the relevant CP347 driver documentation.