Fa il-S a fe , Hig h -S p e e d (1 0 Mb p s ),
S le w -Ra t e -Lim it e d RS -4 8 5 /RS -4 2 2 Tra n s c e ive rs
–MAX3089
B
RECEIVER
OUTPUT
V
ID
R
R
20dB/div
ATE
A
0Hz
100kHz/div
1MHz
Figure 13. Receiver Propagation Delay Test Circuit
Figure 14. Driver Output Waveform and FFT Plot of
MAX3086/MAX3087/MAX3088, and MAX3089 with
SRL = GND, Transmitting a 20kHz Signal
A
A
B
B
20dB/div
20dB/div
0Hz
100kHz/div
1MHz
0Hz
100kHz/div
1MHz
Figure 15. Driver Output Waveform and FFT Plot of
MAX3083/MAX3084/MAX3085, and MAX3089
Figure 16. Driver Output Waveform and FFT Plot of
MAX3080/MAX3081/MAX3082, and MAX3089
with SRL = V
Transmitting a 20kHz Signal
with SRL = Unconnected, Transmitting a 20kHz Signal
CC,
components with large amplitudes are evident. Figure
15 shows the same signal displayed for a MAX3083/
MAX3084/MAX3085, and MAX3089 with SRL = VCC),
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-
tia l for EMI is s ig nific a ntly re d uc e d . Fig ure 16
s hows the s a me s ig na l d is p la ye d 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
/ (10 x 1.5ns/ft)
RISE
where t
is the transmitter’s rise time.
RISE
For e xa mp le , the MAX3080’s ris e time is typ ic a lly
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.
______________________________________________________________________________________ 17
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
