Datasheet

MAX3140
SPI/MICROWIRE-Compatible UART with Integrated
True Fail-Safe RS-485/RS-422 Transceivers
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sis of a 20kHz signal transmitted with SRL = GND. High-
frequency harmonic components with large amplitudes
are evident. Figure 18 shows the same signal for SRL =
V
CC
, transmitting under the same conditions. Figure
18’s high-frequency harmonic components are much
lower in amplitude, compared with Figure 17’s, and the
potential for EMI is significantly reduced. Figure 19
shows the same signal for SRL = unconnected, trans-
mitting under the same conditions. In general, a trans-
mitter’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 · 1.5ns/ft)
where t
RISE
is the transmitter’s rise time.
For example, consider a rise time of 1320ns. This
results in excellent waveforms with a stub length up to
90 feet. A system can work well with longer unterminat-
ed stubs, even with severe reflections, if the waveform
settles out before the UART samples them.
RS-485/RS-422 Transceiver
Low-Power Shutdown Mode
Low-power shutdown mode is initiated by bringing both
RE high and DE low. RE and DE may be driven simulta-
neously; the MAX3140 is guaranteed not to enter shut-
down if RE is high and DE is low for less than 50ns. If
the inputs are in this state for at least 600ns, the device
is guaranteed to enter shutdown.
Enable times t
ZH
and t
ZL
in the Switching Char-
acteristics
tables assume the device was not in a low-
power shutdown state. Enable times t
ZH(SHDN)
and
t
ZL(SHDN)
assume the device was shut down. It takes
drivers and receivers longer to become enabled from
low-power shutdown mode (t
ZH(SHDN)
, t
ZH(SHDN)
) than
from driver/receiver-disable mode (t
ZH
, t
ZL
).
Driver Output Protection
Two mechanisms prevent excessive output current and
power dissipation caused by faults or by bus con-
tention. The first, a foldback current limit on the output
stage, provides immediate protection against short cir-
cuits over the whole common-mode voltage range (see
Typical Operating Characteristics ). The second, a ther-
mal shutdown circuit, forces the driver outputs into a
high-impedance state if the die temperature becomes
excessive.
Line Length vs. Data Rate
The RS-485/RS-422 standard covers line lengths up to
4000 feet. For line lengths greater than 4000 feet, use
the repeater application shown in Figure 20.
Figures 21, 22, and 23 show the system differential volt-
age for the parts driving 4000 feet of 26AWG twisted-
pair wire into 120Ω loads.
100kHz/divO 1MHz
A
20dB/div
MAX3140 FIG18
Figure 18. Driver Output Waveform and FFT Plot of MAX3140
with SRL = V
CC
, Transmitting a 20kHz Signal
100kHz/divO 1MHz
A
20dB/div
MAX3140 FIG19
Figure 19. Driver Output Waveform and FFT Plot of MAX3140
with SRL = Unconnected, Transmitting a 20kHz Signal