UM3487E Datasheet, PDF (11/14 Pages) Union Semiconductor, Inc. – ±15kV ESD-Protected, Fail-Safe, Hot-Swappable Auto Polarity Reversal RS-485 Transceivers

UM3487E
Applications Information
Non-Polarity transceiver
When established the non-polarity RS-485 net, you should pay attention to two conditions. First, a
pair of bias resistance (pull-up to +5V for RS-485 A bus, push-down to GND for RS-485 B bus)
must be required, usually be built in the master node, or independently, 500Ω resistance is
recommended. The other nodes don’t need bias resistance anymore. Second, the transceiver rate
must be higher than 100bps or the maximum transmitting time for low logic should be less than
100ms.
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.
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 10uA 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
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_ZL(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 (see Typical
Operating Characteristics). 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 UM3487E 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.
________________________________________________________________________
http://www.union-ic.com Rev.02 Jan.2014
11/14