IL3485/IL3422
9
NVE Corporation
11409 Valley View Road, Eden Prairie, MN 55344-3617
Phone: (952) 829-9217
Fax: (952) 829-9189
www.IsoLoop.com
NVE Corporation
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 4. 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 3 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
Unterminated
Parallel
Proper termination is imperative when using IL3485 and IL3422 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 (3 V supply) and 500 (5 V supply).
1000
100
40
4000
100
1K
10K
100K
1M
10M
Data Rate (bps)
Cable Length (feet)