Specifications

RS-232/422/485 2-PORT PCI CARD, OPTO-ISOLATED

3.2 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 you to customize this

interface to their specific requirements. Each switch

position corresponds to a specific portion of the

interface. If multiple PCI Cards are configured in a RS-

485 network, only the boards on each end should have

jumpers T, P, and P on. Refer to Table 3-1 for each

position’s operation.

Table 3-1. Jumper settings.

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.