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2-PORT RS-232/422/485 PCI HOST ADAPTER
3. Card Setup
NOTE
In all cases J1x refers to settings for the first port and J2x refer
to settings for the second port.
3.1 RS-485 Enable Modes
RS-485 is ideal for multidrop or network environments. RS-485 requires a tri-state
driver that will allow the electrical presence of the driver to be removed from the
line. The driver is in a tri-state or high-impedance condition when this occurs.
Only one driver may be active at a time, and the other driver(s) must be tri-stated.
The output modem control signal Request To Send (RTS) is typically used to
control the state of the driver. Some communication-software packages refer to
RS-485 as RTS enable or RTS block mode transfer.
One of the unique features of the Adapter is the ability to be RS-485 compatible
without the need for special software or drivers. This ability is especially useful in
Windows, Windows NT, and OS/2 environments, where the lower-level I/O
control is abstracted from the application program. This ability means that the user
can effectively use the Adapter in a RS-485 application with existing (i.e. standard
RS-232) software drivers.
Headers J1B and J2B are used to control the RS-485 mode functions for the
driver circuit. The selections are “RTS” enable (marked “RT”) or “Auto” enable
(marked “AT”). The “Auto” enable feature automatically enables/disables the
RS-485 interface. The “RTS” mode uses the “RTS” modem-control signal to enable
the RS-485 interface and provides backward compatibility with existing software
products.
Position 3 (marked “NE”) of J1B and J2B is used to control the RS-485
enable/disable functions for the receiver circuit and determine the state of the
RS-422/485 driver. The RS-485 “Echo” is the result of connecting the receiver
inputs to the transmitter outputs. Every time a character is transmitted, it is also
received. This can be beneficial if the software can handle echoing (i.e. using
received characters to throttle the transmitter) or it can confuse the system if the
software does not. To select the “No Echo” mode, select the position marked “NE.”
For RS422/530/449 compatibility, remove the jumpers at J1B and J2B.