MAX13442E Datasheet, PDF (15/18 Pages) Maxim Integrated Products – ±15kV ESD-Protected, ±80V Fault-Protected, Fail-Safe RS-485/J1708 Transceivers

±15kV ESD-Protected, ±80V Fault-Protected,
Fail-Safe RS-485/J1708 Transceivers
Applications Information
128 Transceivers on the Bus
The MAX13442E/MAX13443E/MAX13444E transceivers
1/4-unit-load receiver input impedance (48kΩ) allows
up to 128 transceivers connected in parallel on one
communication line. Connect any combination of these
devices, and/or other RS-485 devices, for a maximum
of 32-unit loads to the line.
Reduced EMI and Reflections
The MAX13442E/MAX13444E are slew-rate limited,
minimizing EMI and reducing reflections caused by
improperly terminated cables. Figure 11 shows the dri-
ver output waveform and its Fourier analysis of a
125kHz signal transmitted by a MAX13443E. High-fre-
quency harmonic components with large amplitudes
are evident.
Figure 12 shows the same signal displayed for the
MAX13442E transmitting under the same conditions.
Figure 12’s high-frequency harmonic components are
much lower in amplitude, compared with Figure 11’s,
and the potential for EMI is significantly reduced.
In general, a transmitter’s rise time relates directly to
the length of an unterminated stub, which can be driven
with only minor waveform reflections. The following
equation expresses this relationship conservatively:
length = t_RISE / (10 x 1.5ns/ft)
where t_RISE is the transmitter’s rise time.
For example, the MAX13442E’s rise time is typically
800ns, which results in excellent waveforms with a stub
length up to 53ft. A system can work well with longer
unterminated stubs, even with severe reflections, if the
waveform settles out before the UART samples them.
RS-485 Applications
The MAX13442E/MAX13443E/MAX13444E transceivers
provide bidirectional data communications on multi-
point bus transmission lines. Figure 13 shows a typical
network application circuit. The RS-485 standard cov-
ers line lengths up to 4000ft. To minimize reflections
and reduce data errors, terminate the signal line at both
ends in its characteristic impedance, and keep stub
lengths off the main line as short as possible.
20dB/div
2V/div
0
500kHz/div
5.00MHz
Figure 11. Driver Output Waveform and FFT Plot of the
MAX13443E Transmitting a 125kHz Signal
20dB/div
2V/div
0
500kHz/div
5.00MHz
Figure 12. Driver Output Waveform and FFT Plot of the
MAX13442E Transmitting a 125kHz Signal
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