User's Manual

Planning the Installation

Part No. 430-4G-200-00-000

2-10

2.6.2.1. RS-485 C

ABLE SPECIFICATION

V-Station 4G devices provide a 2-wire, half-duplex RS-485 interface. The main

cable run should be low capacitance, twisted-pair cable, with approximately

120-ohm characteristic impedance. Category-5 rated communications cable is

used in RS-485 networks and its characteristics are defined in Table 2-2 below.

This is the recommended cabling for RS-485 communications. The cable

connection includes a differential line (+ and -) and a GND connection.

Specification Recommendation

Capacitance (conductor to conductor) <20 pF/ft.

Characteristic Impedance 100 – 120 ohms

Nominal DC resistance <100 ohms/1000 ft.

Wire gauge 24 AWG stranded

Conductors/Shielding >2 pair (shielding is recommended)

Table 2-2: Category 5 Cable Characteristics

2.6.2.3. RS-485 N

ETWORK TOPOLOGY

Communication cables for RS-485 should be laid out in a daisy chain

configuration (See Figure 2-3 below). Long stubs or drop downs 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 overloads the driver. Terminating only two endpoints solves the

loading problem, but creates transmission line problems at the unterminated

ends. A true daisy chain configuration avoids these problems.

2.6.2.2. RS-485 C

ABLE LENGTHS

As outlined in the RS-485 specification, the total length of the communication

cable (adding up all of the segments of the run) should not exceed 1200 meters

(4000 feet). Although the RS-485 specification calls for a maximum cable length

of 1200 meters and provides a maximum baud rate well above that of the V-

Station device, a more conservative system should be configured to no more

than 1000 meters and run 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-connections, etc.) to equipment are not

recommended, but if required, should not exceed one foot) 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 device 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.