Chapter 3: Option Selection
To help you locate the jumpers described in this section, refer to the Option Selection Map at the end of
this section. Operation of the serial communications section is determined by jumper installation as
described in the following paragraphs.
For the convenience of the user, instructions for the placement of the jumpers is also silk screened on the
back of the card.

Terminations

A transmission line should be terminated at the receiving end in its characteristic impedance. Installing a
jumper at the locations labeled LDxO applies a 120Ω load across the transmit/receive input/output for
RS485 operation.
Jumpers having to do with the termination of each channel are located near the output connector. They
are labeled by channel. The load jumper is labeled “LD”. The other two jumpers are used to connect the
transmit and receive lines for the two wire RS485 mode.
120
0.01 F

Tx+/Rx

+

Tx-/Rx-

LD

μ

Figure 3-1: Simplified Termination Schematic
In RS485 operations where there are multiple terminals, only the RS485 ports at each end of the network
should have terminating impedance as described above. To so terminate the COM A port, place a jumper
at the location labeled Ch A -LD. To terminate the COM B, COM C, COM D, COM E, COM F and COM H
ports, place jumpers at locations labeled Ch B - LD, Ch C - LD, Ch D - LD, Ch E - LD, Ch F - LD, Ch G -
LD and Ch H - LD respectively.
Also, for RS485 operation, there must be a bias on the TRX+ and TRX- lines. If the card is not to provide
that bias, contact the factory technical support.
Manual LPCI-COM-8SM
12