SN65HVD485E_15 Datasheet, PDF (16/29 Pages) Texas Instruments – Half-Duplex RS-485 Transceiver

SN65HVD485E
SLLS612E – JUNE 2004 – REVISED DECEMBER 2015
www.ti.com
Typical Application (continued)
Receiver failsafe is accomplished by offsetting the receiver thresholds such that the input indeterminate range
does not include zero volts differential. To comply with the RS-422 and RS-485 standards, the receiver output
must output a high when the differential input V_ID is more positive than 200 mV, and it must output a Low when
V_ID is more negative than –200 mV. The receiver parameters that determine the failsafe performance are V_IT+,
V_IT–, and V_hys (the separation between V_IT+ and V_IT–). As shown in the Electrical Characteristics: Receiver table,
differential signals more negative than –200 mV cause a low receiver output, and differential signals more
positive than 200 mV cause a high receiver output.
When the differential input signal is close to zero, it is still above the V_IT+ threshold, and the receiver output is
High. Only when the differential input is more than V_hys below V_IT+ does the receiver output transition to a Low
state. Therefore, the noise immunity of the receiver inputs during bus fault conditions includes the receiver
hysteresis value (V_hys) as well as the value of V_IT+.
10.2.2 Detailed Design Procedure
To protect bus nodes against high-energy transients, the implementation of external transient protection devices
is necessary.
110000nnFF
101k0Ωk
R
RxD
MCU/
RE
UART
DE
DIR
D
TxD
101k0Ωk
5V
100nnFF
V_CC
A
B
GND
R1
TVS
R2
Figure 20. Transient Protection Against ESD, EFT, and Surge Transients
Figure 20 suggests a protection circuit against 10-kV ESD (IEC 61000-4-2), 4-kV EFT (IEC 61000-4-4), and 1-kV
surge (IEC 61000-4-5) transients. Table 3 shows the associated bill of materials.
DEVICE
XCVR
R1, R2
TVS
Table 3. Bill of Materials
FUNCTION
5-V, 10-Mbps RS-485
transceiver
10-Ω, pulse-proof thick-film
resistor
Bidirectional 400-W
transient suppressor
ORDER NUMBER
SN65HVD485E
CRCW0603010RJNEAHP
CDSOT23-SM712
MANUFACTURER
TI
Vishay
Bourns
10.2.2.1 Power Usage in an RS-485 Transceiver
Power consumption is a concern in many applications. Power supply current is delivered to the bus load and to
the transceiver circuitry. For a typical RS-485 bus configuration, the load that an active driver must drive consists
of all of the receiving nodes plus the termination resistors at each end of the bus.
The load presented by the receiving nodes depends on the input impedance of the receiver. The TIA/EIA-485-A
standard defines a unit load as allowing up to 1 mA. With up to 32 unit loads allowed on the bus, the total current
supplied to all receivers can be as high as 32 mA. The SN65HVD485E device is rated as a ½ unit load device,
so up to 64 can be connected on one bus.
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