Impulse 7108 - Manual (Page 5)

Card Setup
Sealevel Systems ULTRA COMM+I.LPCI Page 3
Line Termination
Typically, each end of the RS-485 bus must have line-terminating resistors (RS-422 terminates at the receive end
only). A 120-ohm resistor is across each RS-422/485 input in addition to a 1K-ohm pull-up/pull-down combination
that biases the receiver inputs. Switches SW1 and SW2 allow the user to customize this interface to their specific
requirements. Each switch position corresponds to a specific portion of the interface. If multiple ULTRA
COMM+I.LPCI adapters are configured in a RS-485 network, only the boards on each end should have jumpers T,
P & P ON. Refer to the following table for each position’s operation:
Name Function
T Adds or removes the 120 ohm termination.
P Adds or removes the 1K ohm pull-down resistor in the
RS-422/RS-485 receiver circuit (Receive data only).
P Adds or removes the 1K ohm pull-up resistor in the RS-422/RS-
485 receiver circuit (Receive data only).
L Connects the TX+ to RX+ for RS-485 two wire operation.
L Connects the TX- to RX- for RS-485 two wire operation.
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+I.LPCI 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.LPCI 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).
Caution: Do not set both switches in the ‘On’ position. This could potentially cause damage to the adapter,
and definitely cause a malfunction.
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