Impulse 2801 - Manual (Page 9)

Configuration
Sealevel Systems SeaLINK+8 Page 6
Interface Biasing Configuration
The last four positions for each port are used to select the line termination and
bias settings. Refer to the chart below for specific switch settings of the three
supported electrical interface modes. Note that this chart appears in the silk
screen on the board as well.
PD = 1Kohm pull-down resistor connected to RX-
RS-232 mode = always OFF
RS-422 mode = always ON
RS-485 mode = ON if not provided by another device on the bus
PU = 5.1Kohm pull-up resistor connected to CTS+
RS-232 mode = always OFF
RS-422 mode = always ON
RS-485 mode = always ON
PU = 1Kohm pull-down resistor connected to RX+
RS-232 mode = always OFF
RS-422 mode = always ON
RS-485 mode = ON if not provided by another device on the bus
T = 120ohm terminating resistor connected between RX+ and RX-
RS-232 mode = always OFF
RS-422 mode = always ON
RS-485 mode = ON if physically located at one end of bus
SWITCH
232
INTERFACE BIASING
422
485
OFF OFF
PD PU
ON
ON
ON
ON
OFF OFF
PU T
ON
ON*
ON
ON
* = IF END OF 485 BUS
Figure 3 - Electrical Interface Termination and Biasing Selection
Technical Note
Contrary to some of our other products, there are no provisions on board for
looping TX+ to RX+ and TX- to RX- for 2-wire RS-485 mode. These
connections must be made externally inside the mating DB-9 connector. Refer
to the RS-422 pinout diagram for the appropriate pin numbers.