Impulse 2113 - Manual (Page 10)

Dip Switches (continued)
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. One of
the unique features of SeaLINK USB serial devices is the ability to be RS-485
compatible without the need for special software or drivers (RS-485 auto-enable).
Typically, each end of the RS-485 bus must have a line-terminating resistor (RS-422
terminates the receive end only). A 120
Ω
resistor is across each RS-422/485 input in
addition to a 1K
Ω
pull-up and a 1K
Ω
pull-down combination that biases the receiver
inputs.
Only the ends of an RS-485 network should have the 120
Ω
terminating resistor. The
resistor may be removed or inserted using dip switch #4. This allows you to
customize the electrical interface to your specific requirements.
To add the termination/bias resistors, use dip switches #4, #5, and #6 to select the
‘On’ position, and to remove it, select the ‘Off’ position. If multiple devices are
configured in an RS-485 network, only the devices at each end should have switches
#4, #5, and #6 ‘On’.
The final option that is configurable is the ‘Echo’ option. Two-wire RS-485 connects
the TX
± to the RX±. 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. Switch
#3 is used to control the RS-485 enable/disable functions for the receiver circuit. To
select the ‘No Echo’ mode set switch #3 to ‘On’.
Common SeaLINK device configurations are shown in the examples below.
Example Settings
© Sealevel Systems, Inc. - 8 - SeaLINK DIN Rail USB Serial Manual