IL3285/IL3222

10

NVE Corporation

11409 Valley View Road, Eden Prairie, MN 55344-3617

Phone: (952) 829-9217

www.isoloop.com

iso-info@nve.com

iso-apps@nve.com

Cabling, Data Rate and Terminations

Cabling

Use twisted-pair cable. The cable can be unshielded if it is short (less than 10 meters) and the data rate is slow (less than

100 Kbps). Otherwise, use screened cable with the shield tied to earth ground at one end only. Do not tie the shield to digital

ground. The other end of the shield may be tied to earth ground through an RC network. This prevents a DC ground loop in the

shield. Shielded cable minimizes EMI emissions and external noise coupling to the bus.

Data Rate

The longer the cable, the slower the data rate. The RS-485 bus can transmit ground over 4,000 feet (1,200 meters) or at 10 Mbps,

but not both at the same time. Transducer and cable characteristics combine to act as a filter with the general response shown in

Figure 3. Other parameters such as acceptable jitter affect the final cable length versus data rate tradeoff. Less jitter means better

signal quality but shorter cable lengths or slower data rates. Figure 4 shows a generally accepted 30% jitter and a corresponding

data rate versus cable length.

Figure 3. Cable Length vs. Data Rate (30% jitter).

Terminations

Transmission lines should be terminated to avoid reflections that cause data errors. In RS-485 systems both ends of the bus, not

every node, should be terminated. In RS-422 systems only the receiver end should be terminated.

100 Ω

Unterminated

Parallel

Proper termination is imperative when using IL3285 and IL3222 to minimize reflections. Unterminated lines are only suitable for

very low data rates and very short cable runs, otherwise line reflections cause problems. Parallel terminations are the most popular.

They allow high data rates and excellent signal quality.

Occasionally in noisy environments, fast pulses or noise appearing on the bus lines cause errors. One way of alleviating such errors

without adding circuit delays is to place a series resistor in the bus line. Depending on the power supply, the resistor should be

between 300 Ω (3 V supply) and 500 Ω (5 V supply).

1000

100

40

4000

100

1K

10K

100K

1M

10M

Data Rate (bps)

Cable Length (feet)