il3122 NVE Corp., il3122 Datasheet - Page 8

• Manufacturer Part Number: il3122
• Description: Low Cost Rs-485 And Rs-422 Isolated Transceivers
• Manufacturer: NVE Corp.

Data Rate and Magnetic Field Immunity
It is easier to disrupt an isolated DC signal with an external magnetic field than it is to disrupt an isolated AC signal. Similarly, a
DC magnetic field will have a greater effect on the device than an AC magnetic field of the same effective magnitude. For
example, signals with pulses greater than 100 μs long are more susceptible to magnetic fields than shorter pulse widths. For input
signals faster than 1 MHz, a 1 nF field boost capacitor provides as much as 400 Gauss immunity, while the same input capacitor
might provide just 70 Gauss of immunity at 50 kHz.
Applications Information
RS-485 and RS-422 are differential (balanced) data transmission standards for use over long distances or in noisy environments.
RS-422 is an RS-485 subset, so RS-485 transceivers are also RS-422-compliant. RS-422 is a multi-drop standard allowing only
one driver and up to 10 receivers on each bus (assuming unit load receivers). RS-485 is a true multipoint standard which allows up
to 32 unit load devices (any combination of drivers and receivers) on each bus. To allow for multipoint operation, RS-485 requires
drivers to handle bus contention without damage. Another important advantage of RS-485 is the extended common-mode range
(CMR), which requires driver outputs and receiver inputs withstand +12 V to −7 V. RS-422 and RS-485 are intended for runs as
long as 4,000 feet (1,200 m), so the wide CMR is necessary for ground potential differences, as well as voltages induced in the
cable by external fields.
Receiver Features
IL3000 transceivers have differential input receivers for maximum noise immunity and common-mode rejection. Input sensitivity
is ±200 mV as required by the RS-422 and RS-485 specifications. The receivers include a “fail-safe if open” function that
guarantees a high level receiver output if the receiver inputs are unconnected (floating). Receivers easily meet the data rates
supported by the corresponding driver. IL3000-Series receiver outputs have tri-state capabilities with active low RE inputs.
Driver Features
The RS485/422 driver is a differential output device that delivers at least 1.5 V across a 54 Ω load (RS-485), and at least 2 V
across a 100 Ω load (RS-422). The driver features low propagation delay skew to maximize bit width and minimize EMI. IL3222
and IL3285 drivers have tri-state capability with an active high DE input.
Cabling, Data Rate and Terminations
Cabling:
Use twisted-pair cable. The cable can be unshielded if it is short (<10 m) and the data rate is slow (<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 m) or at 10Mbps, 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 3 shows a generally accepted 30% jitter and a corresponding
data rate versus cable length.
Method
Field applied in direction H1
Field applied in direction H2
Field applied in any direction but with field
booster capacitor (1 nF) in circuit
Table 1. Magnetic Immunity
NVE Corporation
11409 Valley View Road, Eden Prairie, MN 55344-3617
Approximate Immunity
±20 Gauss
±70 Gauss
±250 Gauss
Phone: [phone]
8
Fax: [phone]
Immunity Description
A DC current of 16 A flowing in a conductor
1 cm from the device could cause disturbance
A DC current of 56 A flowing in a conductor
1 cm from the device could cause disturbance
A DC current of 200 A flowing in a conductor
1 cm from the device could cause disturbance
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