Datasheet

SN65HVD30 – SN65HVD35

SLLS665I –SEPTEMBER 2005–REVISED APRIL 2010

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Typical RS-485 twisted-pair cable has capacitance of approximately 50 pF/meter. Therefore it is expected that 10

meters of cable would provide sufficient capacitance to prevent this latch-up condition.

The –7 to +12V common mode range specified by RS-485 is intended to allow communication between

transceivers separated by significant distances, when ground offsets may occur due to temporary current surges,

electrical noise, etc. In those circumstances, the inherent cable needed to connect separated transceivers will

ensure that the conditions above do not occur. For transceiver separated by only a short cable length, or

backplane applications, it would be unusual for there to be a steady-state negative common-mode voltage. It is

possible for a negative power supply to be shorted to the bus lines due to mis-wiring or cable damage, however,

this is a different root cause fault, and robust devices such as the HVD178x family should be used for surviving

power supply or mis-wiring faults.

The 250 mA current limit in the RS-485 standard is intended to prevent damage caused by data contention on

the bus; that is, in the event that two or more transceivers drive the bus to different states at the same time.

These devices will not be damaged under these conditions, because all RS-485 drivers have output impedance

sufficient to prevent the direct connection condition stated above. Typical RS-485 driver output impedance is on

the order of 10 to 30 Ω.

HOT-PLUGGING

These devices are designed to operate in hot swap or hot pluggable applications. Key features for hot-pluggable

applications are power-up, power-down glitch-free operation, default disabled input/output pins, and receiver

failsafe. As shown in Figure 24, an internal Power-On Reset circuit keeps the driver outputs in a high-impedance

state until the supply voltage has reached a level at which the device will reliably operate. This ensures that no

spurious bits are transmitted on the bus pin outputs as the power supply turns on or turns off.

As shown in the device FUNCTION TABLES, the enable inputs have the feature of default disable on both the

driver enable and receiver enable. This ensures that the device will neither drive the bus nor report data on the R

pin until the associated controller actively drives the enable pins.

RECEIVER FAILSAFE

The differential receiver is failsafe to invalid bus states caused by open bus conditions such as, a disconnected

connector, shorted bus conditions caused by damaged cabling, or idle bus conditions that occur when no driver

is actively driving a valid RD-485 bus state on the network. In any of these cases, the differential receiver will

output a failsafe HIGH state, so that small noise signals do not cause spurious transitions at the receiver output.

SAFE OPERATION WITH BUS CONTENTION

These devices incorporate a driver current limit of 250 mA across the RS-485 common-mode range of –7 V to

+12 V. As stated in the "Application Guidelines for TIA/EIA-485-A"

(1)

this sets a practical limitation to prevent

damage during bus contention events. Contention can occur during system initialization, during system faults, or

whenever two or more drivers are active at the same time.

Figure 14 shows a 2-node system to demonstrate bus contention by forcing both drivers to be active in opposing

states.

(1) TIA/EIA Telecommunications System Bulletin TSB89, "Application Guidelines for TIA/EIA-485-A"

Product Folder Link(s) :SN65HVD30 – SN65HVD35