XR3170E Datasheet, PDF (17/24 Pages) Exar Corporation – 3.3V High Speed RS-485/RS-422 TRANSCEIVERS WITH 1/8 UNIT LOAD

REV. 1.0.0
XR3170E-3178E
3.3V HIGH SPEED RS-485/RS-422 TRANSCEIVERS WITH 1/8^th UNIT LOAD
IEC 61000-4-2
The IEC 61000-4-2 standard covers ESD testing and performance of finished equipment. However, it does not
specifically refer to integrated circuits. The XR3170E family helps you design equipment to meet
IEC 61000-4-2, without sacrificing board-space and cost for external ESD-protection components.
The major differences between tests done using the Human body model and IEC 61000-4-2 is a higher peak
current in IEC 61000-4-2. Series resistance is lower in the IEC 61000-4-2 model. Hence, the ESD withstand
voltage measured to IEC 61000-4-2 is generally lower than that of human body model.
The air-gap test involves approaching the device with a charged probe. The contact discharge method
connects the probe to the device before the probe is energized.
MACHINE MODEL
The machine model for ESD tests all pins using a 200pF storage capacitor and zero discharge resistance. The
objective is to emulate the stress caused when I/O pins are contacted by handling equipment during test and
assembly.
256 TRANSCEIVERS ON THE BUS
The standard RS-485 receiver input impedance is 12k Ohms (1 unit load). A standard driver can drive up to 32
unit loads. The XR3170E family of transceivers have only a 1/8^th unit load receiver input impedance of 96k
Ohms, thereby allowing eight times as many, up to 256, transceivers to be connected in parallel on a
communication line. Any combination of these devices and other RS-485 transceivers up to a total of 32 unit
loads may be connected to the line.
LOW POWER SHUTDOWN MODE
Low-power shutdown mode is initiated by bringing both RE high and DE low simultaneously. While in shutdown
devices draw less than 1µA of supply current. DE and RE may be tied together and driven by a single control
signal. Devices 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 will enter shutdown.
Enable times t_ZH and t_ZL apply when the part is not in low-power shutdown state. Enable times t_ZH(SHDN) and
t_ZL(SHDN) apply when the parts are shutdown. The drivers and receivers take longer to become enabled from
low-power shutdown t_ZH(SHDN) and t_ZL(SHDN) than from driver / receiver disable mode (t_ZH and t_ZL).
DRIVER OUTPUT PROTECTION
Two mechanisms prevent excessive output current and power dissipation caused by faults or by bus
contention. First, a driver current limit on the output stage provides immediate protection against short circuits
over the whole common-mode voltage range. Second, a thermal-shutdown circuit forces the driver outputs into
a high-impedance state if junction temperature becomes excessive.
LINE LENGTH
The RS-485/RS-422 standard covers line lengths up to 4000ft. Maximum achievable line length is a function of
signal attenuation and noise. Termination prevents signal reflections by eliminating the impedance mismatches
on a transmission line. Line termination is generaly used if rise and fall times are shorter than the round-trip
signal propagation time
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