Impulse 7203 - Manual (Page 7)

Card Setup
Sealevel Systems ULTRA COMM+2I.PCI Page 4
RS-485 Enable Modes
RS-485 is ideal for multi-drop 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 ULTRA COMM+2I.PCI is the ability to be
RS-485 compatible without the need for special software or drivers. This ability
is especially useful in Operating Systems where the lower level I/O control is
abstracted from the application program. This ability means that the user can
effectively use the ULTRA COMM+2I.PCI in an RS-485 application with
existing (i.e. standard RS-232) software drivers.
Switches SW3 and SW4 are used to control the RS-485 mode functions for the
driver circuit. The selections are ‘RTS’ enable (silk-screen ‘RT’ switch position
4) or ‘Auto’ enable (silk-screen ‘AT’ switch position 3). 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.
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. Position 9 of SW1 and SW2 is used to control the RS-485
enable/disable functions for the receiver circuit. To select the ‘No Echo’ mode
place switch position 9 to the ‘On’ position (see Figure 5).
Figure 6 - RS-485 Enable modes
1 2 3 4
ON
OFF
D
1 D
4 A
T R
T
1 2 3 4
ON
OFF
SW3
SW4
RS-232/422
Default
1 2 3 4
ON
OFF
D
1 D
4 A
T R
T
1 2 3 4
ON
OFF
SW3
SW4
RS-485
Auto
Enabled