Datasheet

components with large amplitudes are evident. Figure
15 shows the same signal displayed for a MAX3083/
MAX3084/MAX3085, and MAX3089 with SRL = V
CC),
transmitting under the same conditions. Figure 15’s
high-frequency harmonic components are much lower
in amplitude, compared with Figure 14’s, and the poten-
tial for EMI is significantly reduced. Figure 16
shows the same signal displayed for a MAX3080/
MAX3081/MAX3082, and MAX3089 with SRL = uncon-
nected, transmitting under the same conditions. Figure
16’s high-frequency harmonic components are even
lower.
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 MAX3080’s rise time is typically
1320ns, which results in excellent waveforms with a stub
length up to 90 feet. A system can work well with longer
unterminated stubs, even with severe reflections, if the
waveform settles out before the UART samples them.
Fail-Safe, High-Speed (10Mbps),
Slew-Rate-Limited RS-485/RS-422 Transceivers
R
B
RECEIVER
OUTPUT
ATE
A
V
ID
R
Figure 13. Receiver Propagation Delay Test Circuit
100kHz/div0Hz 1MHz
20dB/div
MAX3080/3089 FIG-14
Figure 14. Driver Output Waveform and FFT Plot of
MAX3086/MAX3087/MAX3088, and MAX3089 with
SRL = GND, Transmitting a 20kHz Signal
100kHz/div0Hz 1MHz
B
A
20dB/div
MAX3080/3089 FIG-15
Figure 15. Driver Output Waveform and FFT Plot of
MAX3083/MAX3084/MAX3085, and MAX3089
with SRL = V
CC,
Transmitting a 20kHz Signal
100kHz/div0Hz 1MHz
B
A
20dB/div
MAX3080/3089 FIG-16
Figure 16. Driver Output Waveform and FFT Plot of
MAX3080/MAX3081/MAX3082, and MAX3089
with SRL = Unconnected, Transmitting a 20kHz Signal
Maxim Integrated
17
MAX3080–MAX3089