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MAX1481 Datasheet(PDF) 10 Page - Maxim Integrated Products

Part No. MAX1481
Description  Software-Selectable, Half-/Full-Duplex, Slew-Rate-Limited, 12Mbps, RS-485/RS-422 Transceivers in uMAX Package
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com

MAX1481 Datasheet(HTML) 10 Page - Maxim Integrated Products

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Software-Selectable, Half-/Full-Duplex, Slew-Rate-Limited,
12Mbps, RS-485/RS-422 Transceivers in µMAX Package
_______________Detailed Description
The MAX1481/MAX1484/MAX1485/MAX1486 high-
speed transceivers for RS-485/RS-422 communication
contain one driver and one receiver. The MAX1481/
MAX1485 feature reduced-slew-rate drivers that mini-
mize EMI and reduce reflections caused by improperly
terminated cables, allowing error-free data transmission
up to 250kbps. The MAX1484/MAX1486 driver slew
rates are not limited, making transmission speeds up to
12Mbps possible.
These transceivers are designed to operate on a +5V
single supply and typically draw 300µA of supply cur-
rent when unloaded or fully loaded with the drivers dis-
abled. The MAX1481 has a shutdown mode in which
supply current is typically reduced to 0.1µA.
Drivers are output short-circuit current limited and are
protected against excessive power dissipation by ther-
mal-shutdown circuitry that places the driver outputs
into a high-impedance state.
All devices have a 1/8-unit-load receiver input imped-
ance that allows up to 256 transceivers on the bus. The
MAX1481/MAX1484 are designed for full-duplex com-
munications. The H/F pin on the MAX1485/MAX1486
allows the user to select between half-duplex or full-
duplex operation (Figure 4).
Half-/Full-Duplex Mode Operation
The MAX1484/MAX1485 can operate in full- or half-
duplex mode. Drive the H/F pin low or connect it to
GND for full-duplex operation, or drive it high for half-
duplex operation. In full-duplex mode, the pin configu-
ration of the driver and receiver is the same as a
MAX1481 (Figure 1).
__________Applications Information
256 Transceivers on the Bus
The standard RS-485 receiver input impedance is 12k
(1-unit load), and the standard driver can drive up to
32-unit loads. The MAX1481/MAX1484/MAX1485/
MAX1486 transceivers have a 1/8-unit-load receiver
input impedance (96k
Ω), allowing up to 256 trans-
ceivers to be connected in parallel on one communica-
tion line. Connect any combination of these devices
and/or other RS-485 transceivers totaling 32-unit loads
or less.
Reduced EMI and Reflections
The MAX1481/MAX1485 are slew-rate limited, minimiz-
ing EMI and reducing reflections caused by improperly
terminated cables. Figure 14 shows the driver output
waveform and its Fourier analysis of a 20kHz signal
transmitted by a MAX1484. High-frequency harmonic
components with large amplitudes are evident. Figure
15 shows the same signal displayed for a MAX1481
transmitting under the same conditions. Figure 15’s
high-frequency harmonic components are much lower
in amplitude compared to Figure 14’s, significantly
reducing potential EMI.
In general, a transmitter’s rise time relates directly to
the length of an unterminated stub, which can be dri-
ven with only minor waveform reflections. The following
equation expresses this relationship conservatively:
Length = tRISE / (10 x 1.5ns/ft)
where tRISE is the transmitter’s rise time.
For example, the MAX1481’s rise time is typically
500ns, which results in excellent waveforms with a stub
length up to 33 feet. A system may work well with
longer unterminated stubs, even with severe reflec-
tions, if the waveform settles out before the UART
samples them.
Figure 4. MAX1485/MAX1486 Functional Diagram

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