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http://www.union-ic.com

Rev.03Jun.2013

9/12

UM3085E/UM3088E

Applications Information

256 Transceivers on the Bus

The standard RS-485 receiver input impedance is 12kΩ (one unit load), and the standard driver

can drive up to 32 unit loads. The Union family of transceivers have 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-485 transceivers with a

total of 32 unit loads or less can be connected to the line.

Reduced EMI and Reflections

The UM3085E is slew-rate limited, minimizing EMI and reducing reflections caused by

improperly terminated cables. 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:

A system can work well with longer unterminated stubs, even with severe reflections, if the

waveform settles out before the UART samples them.

Low-Power Shutdown Mode

Low-power shutdown mode is initiated by bringing both RE

______

high and DE low. In shutdown, the

device typically draws only 2nA of supply current. RE

______

and DE may be driven simultaneously;

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

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

thermal shutdown circuit, forces the driver outputs into a high-impedance state if the die

temperature becomes excessive.

Line Length vs. Data Rate

The RS-485/RS-422 standard covers line lengths up to 4000 feet. For line lengths greater than

4000 feet, repeater is required.

Typical Applications

The UM3085E, UM3088E transceivers are designed for bidirectional data communications on

multipoint bus transmission lines. To minimize reflections, the line should be terminated at both

ends in its characteristic impedance, and stub lengths off the main line should be kept as short as

possible.