User's Manual

COMPONENT INSTALLATION

7.3.1 RS-485 Cable Lengths

The total length of the communication cable (adding up all of the segments of the run) should not exceed 1200

meters (4000 feet) as outlined in the specification for RS-485. Although the RS-485 specification calls out a

maximum cable length of 1200 meters and a maximum baud rate well above that of the PIV-Station reader, a

more conservative system should be configured for no more than 1000 meters and running at a baud rate of

9600 bits per second. After the network is configured and is running in a stable manner, the baud rate can be

increased if faster network communications are desired.

Drops (down leads, stubs, T’s, T-connections) to equipment are not recommended, but if required, should not

exceed 0.5 meters (about 1.5 feet) and should use the same cable recommended above. On a long stub, a

signal that travels down the wire reflects to the main line after hitting the input impedance of the reader at the

end. This impedance is high compared with that of the cable and the net effect is degradation of signal quality

on the bus.

7.3.2 RS-485 Network Topology

Communication cables for RS-485 should be laid out in a daisy chain (See Figure 4). Long stubs or drop downs

(See 7.5 Cable lengths) and the star configuration should be avoided because they create discontinuities and

degrade signal quality. The star configuration usually does not provide a clean signaling environment even if the

cable runs are all of equal length. The star configuration also presents a termination problem, because

terminating every endpoint will overload the driver. Terminating only two endpoints solves the loading problem

but creates transmission-line problems at the un-terminated ends. A true cascade or daisy chain connection

avoids these problems.

Figure 4: Network Topologies - Star and Cascade (Daisy Chain)

7.3.3 Extending the RS-485 Specification

Some systems require longer distances or higher numbers of nodes than supported by RS-485. Repeaters are

commonly used to overcome these barriers. An RS-485 repeater can be placed in a system to divide the load

into multiple segments. Each refreshed signal is capable of driving another 1200 meters (4000 feet) of cable

and an additional 31 RS-485 reader loads. The PIV-Station readers represent a single reader load to the

transmission line.

7.4 Ethernet Networks

If your system will be configured for use over Ethernet, the wiring will be slightly different. Communication

cables for Ethernet logically form a straight line bus but the more devices on that bus the less efficient the

network becomes due to increased collisions and the weaker the signal will get over distance. Repeaters can

be used to boost the signal strength however a better solution is to place switches at intermediate positions

along the bus. The most common Ethernet topology in use today is the star configuration (See Figure 4) with a

hub or switch in the center.