Detailed Description
The MAX3440E–MAX3444E fault-protected transceivers
for RS-485/RS-422 and J1708 communication contain
one driver and one receiver. These devices feature fail-
safe circuitry, which guarantees a logic-high receiver
output when the receiver inputs are open or shorted, or
when they are connected to a terminated transmission
line with all drivers disabled (see the
True Fail-Safe
section). All devices have a hot-swap input structure
that prevents disturbances on the differential signal
lines when a circuit board is plugged into a hot back-
plane (see the
Hot-Swap Capability
section). The
MAX3440E/MAX3442E/MAX3444E feature a reduced
slew-rate driver that minimizes EMI and reduces reflec-
tions caused by improperly terminated cables, allowing
error-free data transmission up to 250kbps (see the
Reduced EMI and Reflections
section). The MAX3441E/
MAX3443E drivers are not slew-rate limited, allowing
transmit speeds up to 10Mbps.
Driver
The driver accepts a single-ended, logic-level input
(DI) and transfers it to a differential, RS-485/RS-422
level output (A and B). Deasserting the driver enable
places the driver outputs (A and B) into a high-imped-
ance state.
Receiver
The receiver accepts a differential, RS-485/RS-422
level input (A and B), and transfers it to a single-ended,
logic-level output (RO). Deasserting the receiver enable
places the receiver inputs (A and B) into a high-imped-
ance state (see Tables 1–7).
Low-Power Shutdown
(MAX3442E/MAX3443E/MAX3444E)
The MAX3442E/MAX3443E/MAX3444E offer a low-power
shutdown mode. Force DE low and RE high to shut down
the MAX3442E/MAX3443E. Force DE and RE high to
shut down the MAX3444E. A time delay of 50ns prevents
the device from accidentally entering shutdown due to
logic skews when switching between transmit and
receive modes. Holding DE low and RE high for at least
800ns guarantees that the MAX3442E/MAX3443E enter
shutdown. In shutdown, the devices consume a maxi-
mum 20µA supply current.
±
60V Fault Protection
The driver outputs/receiver inputs of RS-485 devices in
industrial network applications often experience voltage
faults resulting from shorts to the power grid that
exceed the -7V to +12V range specified in the EIA/TIA-
485 standard. In these applications, ordinary RS-485
devices (typical absolute maximum -8V to +12.5V)
require costly external protection devices. To reduce
system complexity and eliminate this need for external
protection, the driver outputs/receiver inputs of the
MAX3440E–MAX3444E withstand voltage faults up to
±60V with respect to ground without damage.
Protection is guaranteed regardless whether the device
is active, shut down, or without power.
True Fail-Safe
The MAX3440E–MAX3444E use a -50mV to -200mV
differential input threshold to ensure true fail-safe
receiver inputs. This threshold guarantees the receiver
outputs a logic-high for shorted, open, or idle data
lines. The -50mV to -200mV threshold complies with
the ±200mV threshold EIA/TIA-485 standard.
MAX3440E–MAX3444E
±15kV ESD-Protected, ±60V Fault-Protected,
10Mbps, Fail-Safe RS-485/J1708 Transceivers
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