MAX13448E
±80V Fault-Protected Full-Duplex
RS-485 Transceiver
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Hot-Swap Input Circuitry
The enable inputs feature hot-swap capability. At the
input there are two NMOS devices, M1 and M2 (Figure
9). When V
CC
ramps from zero, an internal 7µs timer
turns on M2 and sets the SR latch that also turns on M1.
Transistor M2, a 1.5mA current sink, and M1, a 100µA
current sink, pull DE to GND through a 5kΩ resistor. M2
is designed to pull DE to the disabled state against an
external parasitic capacitance up to 100pF that can
drive DE high. After 7µs, the timer deactivates M2 while
M1 remains on, holding DE low against three-state leak-
ages that can drive DE high. M1 remains on until an
external source overcomes the required input current.
At this time, the SR latch resets and M1 turns off. When
M1 turns off, DE reverts to a standard, high-impedance
CMOS input.
Applications Information
256 Transceivers on the Bus
The RS-485 standard specifies the load each receiver
places on the bus in terms of unit loads. An RS-485
compliant transmitter can drive 32 one-unit loads when
used with a 120Ω cable that is terminated on both ends
over a common-mode range of -7V to +12V. The
MAX13448E is specified as 1/8 unit loads. This means
a compliant transmitter can drive up to 256 MAX13448E
devices. Reducing the common mode and/or changing
the characteristic impedance of the cable changes the
maximum number of receivers that can be used. Refer
to the TIA/EIA-485 specification for further details.
Proper Termination and Cabling/Wiring
Configurations
When the data rates for RS-485 are high relative to its
cable lengths, the system is subject to proper transmis-
sion line design. In most cases, a single, controlled-
impedance cable or trace should be used and should be
properly terminated on both ends with the characteristic
impedance of the cable/trace. RS-485 transceivers
should be connected to the cable/traces with minimum
length wires to prevent stubs. Star configurations and
improperly terminated cables can cause data loss. Refer
to the
Applications
section of the Maxim website or to
TIA/EIA publication TSB89 for further information.
Reduced EMI and Reflections
The MAX13448E features reduced slew-rate drivers
that minimize EMI and reduce reflections caused by
improperly terminated cables, allowing error-free data
transmission up to 500kbps.
Line Length
The Telecommunications Industry Association (TIA)
publishes the document TSB-89:
Application
Guidelines for TIA/EIA-485-A
that is a good reference
for determining maximum data rate vs. line length.
Typical Applications
The MAX13448E transceivers are designed for bidirec-
tional data communications on multipoint bus transmis-
sion lines. Figure 10 shows a typical network application
circuit. To minimize reflections, terminate the line at both
ends in its characteristic impedance, and keep stub
lengths off the main line as short as possible.
