Options 2 and 3 Twisted pair transceivers

The RS-485 standard further specifies that network cabling have a characteristic impedance of approximately 120W. The use of coaxial or twinaxial cable is not recommended. VISTA Electronics recommends using a high quality, low loss polyethylene-insulated twisted pair wire (such as Belden 9841) for optimum results, although less expensive PVC-insulated wire may be used as well.

LM 104-P50-1

NETW ORK

R11

10W

120W

Figure 2. Termination of an LM104-P50-1 RS-485 network node.

Options 2 and 3 – Twisted pair transceivers

Twisted pair transceivers are connected in a bus topology, and only the nodes at each end of the bus need to be terminated. If the LM104-P50 board is at either end of the bus, enable the built-in termination by installing a shorting jumper at location JP3. If the LM104-P50 is not at either end of the bus, leave JP3 open. The factory default is no termination – JP3 is open.

Twisted pair transceivers may be used with only certain types of wiring. 78Kbps networks (Option

2)may use 22AWG (0.65mm) Level IV cable types only. Please see Appendix E for a partial list of Level IV cable suppliers.

1250Kbps networks (Option 3) may use either 22AWG (0.65mm) Level IV cable or the more common Category 5 (TIA 586A) cable.

Please note that there are distance and other cabling considerations that are far beyond the scope of this manual. VISTA Electronics recommends consulting Echelon Engineering Bulletin #05-0023- 01, “Junction Box and Wiring Guidelines” for additional information.