UM488_15 Datasheet, PDF (10/17 Pages) Union Semiconductor, Inc. – Enhanced Slew-Rate Limiting Facilitates Error-Free Data Transmission

UM488/UM491
Detailed Description
The UM488/UM491 high-speed transceivers for RS-422 communication contain one driver and
one receiver. The device features 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.
The UM488/UM491 offer higher driver output slew-rate limits, allowing transmit speeds up to
2.5Mbps.
The UM488/UM491 is full-duplex transceiver. It operates from a single +5V supply. Drivers are
output short-circuit current limited. Thermal shutdown circuitry protects drivers against excessive
power dissipation. When activated, the thermal shutdown circuitry places the driver outputs into a
high- impedance state.
Receiver Input Filtering
The receivers of the UM488/UM491 incorporate input filtering in addition to input hysteresis.
This filtering enhances noise immunity with differential signals that have very slow rise and fall
times. Receiver propagation delay increases by 20% due to this filtering.
Fail-Safe
The UM488/UM491 guarantees a logic-high receiver output when the receiver inputs are shorted
or open, or when they are connected to a terminated transmission line with all drivers disabled.
This is done by setting the receiver threshold between -50mV and -200mV. If the differential
receiver input voltage (A-B) is greater than or equal to -50mV, RO is logic high. If A-B is less
than or equal to -200mV, RO is logic low. In the case of a terminated bus with all transmitters
disabled, the receiver’s differential input voltage is pulled to 0V by the termination. With the
receiver thresholds of the UM488/UM491, this results in a logic high with a 50mV minimum
noise margin. Unlike previous fail-safe devices, the -50mV to -200mV threshold complies with
the ±200mV EIA/TIA-422 standard.
±15kV ESD Protection
As with all Union devices, ESD-protection structures are incorporated on all pins to protect
against electrostatic discharges encountered during handling and assembly. The driver outputs and
receiver inputs of the UM488/UM491 have extra protection against static electricity. Union’s
engineers have developed state-of-the-art structures to protect these pins against ESD of ±15kV
without damage.
The ESD-protected pins are tested with reference to the ground pin in a powered-down condition.
They are tested to ±15kV using the Human Body Model.
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http://www.union-ic.com Rev.05 Dec.2014
10/17