Cable Grounding and Termination, Option 1 RS-485

Cable Grounding and Termination

To maintain data transmission performance, network cabling must be terminated correctly. Termination requirements vary depending on the type of cable used for network connections – shielded or unshielded twisted pair - and the type of transceiver used.

Cable Grounding

The majority of LonWorks networks use twisted-pair wiring to interconnect the nodes, either shielded twisted pair (STP) or unshielded twisted pair (UTP). If shielded twisted pair wiring is used, the shield of the cable should be grounded as illustrated in Figure 1.

Shielded Twisted Pair

SHIELD GROUNDING CIRCUIT

470KW, 5%, ¼W

100nF, 100V, 10% Metallized Polyester

Figure 1. Cable Shield Grounding Circuit.

The 470KW resistor safely bleeds off any static charge on the cable’s shield, and the capacitor helps avoid DC and 50/60Hz ground paths from being formed through the shield. The cable shield should be grounded at least once per segment, and at each node if possible.

Cable Wiring and Termination

Termination of the network depends upon the type of transceiver used. In some installations, the termination circuit included on the LM104-P50 may be used; in others, it should not be used. The type of wire that may be used, per Echelon’s specifications, also varies by transceiver type.

Option 1 – RS-485

The RS-485-standard transceiver used in Option 1 boards specifies that a 120W, ¼ W resistor should be installed across the network connections (pins 2 and 3 on P3) at every node (see Figure 2).