Datasheet

V
ID
- mV
R
-20-80 0
V
HYS-min
80
V
noise-max
= 160mVpp
60mV
RxD
TxD
DRV
MCU
R
RE
DE
D
A
B
Vcc
GND
1
2
3
4
7
6
5
3.3V
10k
HVD82
8
5V
0.1µF
BAS70
RCV
SN65HVD82
www.ti.com SLLSED6 –OCTOBER 2012
3V–5V INTERFACE
Interfacing the SN65HVD82 to a 3V controller is easy. Because the 5V logic inputs of the transceiver accept 3V
input signals they can be directly connected to the controller I/O. The 5V receiver output, R, however must be
level-shifted via a Schottky diode and a 10k resistor to connect to the controller input. When R is high, the diode
is reverse biased and the controller supply potential lies at the controller input. When R is low, the diode is
forward biased and conducts. In this case only the diode forward voltage of 0.2V lies at the controller input.
Figure 17. 3V–5V Interface
NOISE IMMUNITY
The input sensitivity of a standard RS-485 transceiver is ±200 mV. When the differential input voltage, V
ID
, is
greater than +200 mV, the receiver output turns high, for V
ID
≤ 200 mV the receiver outputs low. Bus voltages in
between these levels can cause the receiver output to go high, or low, or even toggle between logic states. Small
bus voltages however occur every time during the bus access hand-off from one driver to the next as the low-
impedance termination resistors reduce the bus voltage to zero. To prevent receiver output toggling during bus
idling, and thus increasing noise immunity, external bias resistors must be applied to create a bus voltage that is
greater than the input sensitivity plus any expected differential noise.
Figure 18. SN65HVD82 Noise Immunity
The SN65HVD82 transceiver circumvents idle-bus and differential noise issues by providing a positive input
threshold of –20 mV and a typical hysteresis of 60 mV. In the case of an idle-bus condition therefore, a
differential noise voltage of up to 160 mV
PP
can be present without causing the receiver output to change states
from high to low. This increased noise immunity eliminates the need for idle-bus failsafe bias resistors and allows
for long haul data transmissions in noisy environment.
TRANSIENT PROTECTION
The bus terminals of the SN65HVD82 transceiver family possess on-chip ESD protection against ±15 kV human
body model (HBM) and ±12 kV IEC61000-4-2 contact discharge. As stated in the IEC 61000-4-2 standard,
contact discharge is the preferred test method; although IEC air-gap testing is less repeatable than contact
testing, air discharge protection levels are inferred from the contact discharge test results. The IEC-ESD test is
far more severe than the HBM-ESD test. The 50% higher charge capacitance, CS, and 78% lower discharge
resistance, RD of the IEC-model produce significantly higher discharge currents than the HBM-model.
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