Card
Setup
Sealevel Systems
ULTRA
COMM+I.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
R
equest
T
o
S
end (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+I.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+I.PCI
in an RS-485 application with
existing (i.e. standard RS-232) software drivers.
Switch SW2 is 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 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
SW2
RS-232/422
Default
1 2 3 4
ON
OFF
D
1
D
4
A
T
R
T
SW2
RS-485
Auto
Enabled
