UM3491 Datasheet, PDF (11/14 Pages) Union Semiconductor, Inc. – +3.3V Low Power，±15kV ESD-Protected, Fail-Safe, RS-422 Transceivers in SOP14/DIP14

UM3491
Applications Information
256 Transceivers on the Bus
The standard RS-422 receiver input impedance is 12kΩ (one-unit load), and the standard driver
can drive up to 32 unit loads. The UM3491 has a 1/8-unit-load receiver input impedance (96kΩ),
allowing up to 256 transceivers to be connected in parallel on one communication line. Any
combination of these devices and/or other RS-422 transceivers with a total of 32 unit loads or less
can be connected to the line.
Reduced EMI and Reflections
The UM3491 is slew-rate limited, minimizing EMI and reducing reflections caused by improperly
terminated cables. It’s high-frequency harmonic components are much lower in amplitude, and
the potential for EMI is significantly reduced.
In general, a transmitter’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.
Low-Power Shutdown Mode
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Low-power shutdown mode is initiated by bringing both RE high and DE low. In shutdown, the
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devices typically draw only 1nA of supply current. RE and DE may be driven simultaneously;
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the parts are guaranteed not to enter shutdown if RE is high and DE is low for less than 50ns. If
the inputs are in this state for at least 600ns, the parts are guaranteed to enter shutdown.
Enable times t _ZH and t _ZL in the Switching Characteristics tables assume the part was not in a
low-power shutdown state. Enable times t_ZH(SHDN) and t_ZL(SHDN) assume the parts were 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 contention. The first, a foldback current limit on the output stage, provides immediate
protection against short circuits over the whole common-mode voltage range. 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-422 standard covers line lengths up to 4000 feet. For line lengths greater than 4000 feet,
use the repeater application shown in Figure 12.
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http://www.union-ic.com Rev.02 Jun.2012
11/14