VP30 Marking, SN65HVD30D datasheet(21/46 Pages) TI2

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If only the receiver is re-enabled ( RE transitions to low) the receiver output is driven according to the state of the

bus inputs (A and B) after the enable times given by tPZH2 and tPZL2 in the receiver switching characteristics. If

there is no valid state on the bus the receiver responds as described in the failsafe operation section.

If both the receiver and driver are re-enabled simultaneously, the receiver output is driven according to the state

of the bus inputs (A and B) and the driver output is driven according to the D input. Note that the state of the

active driver affects the inputs to the receiver. Therefore, the receiver outputs are valid as soon as the driver

outputs are valid.

8.3.2 Driver Output Current Limiting

The RS-485 standard (ANSI/TIA/EIA-485-A or equivalently ISO 8482) specifies a 250-mA driver output current

limit to prevent damage caused by data contention on the bus. That applies in the event that two or more

transceivers drive the bus to opposing states at the same time. The SN65HVD3x family of devices includes

current-limiting circuitry that prevents damage under these conditions.

Note

This current limit prevents damage during the bus contention, but the logic state of the bus can be

indeterminate as specified by the standard, so communication errors can occur.

In a specific combination of circumstances, a condition can occur in which current through the bus pin exceeds

the 250-mA limit. This combination of conditions is not normally included in RS-485 applications:

• Loading capacitance on the pin is less than 500 pF

• The bus pin is directly connected to a voltage more negative than –1 V

• The device is supplied with VCC equal to or greater than 3.3 V

• The driver is enabled

• The bus pin is driving to the logic high state

In these specific conditions, the normal current-limit circuitry and thermal-shutdown circuitry does not limit or

shutdown the current flow. If the current is allowed to continue, the device heats up in a localized area near the

driver outputs, and the device can be damaged.

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

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

The –7 to +12-V 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, and so on. Under those circumstances, the inherent cable needed to connect separated

transceivers ensures that the conditions previously listed do not occur. For a transceiver separated by only a

short cable length or backplane applications, it is 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 miswiring or cable

damage; however, this is a different root cause fault, and robust devices such as the SN65HVD178x family

should be used for surviving power supply or miswiring 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 are not damaged under these conditions because all RS-485 drivers have output impedance

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

on the order of 10 Ω to 30 Ω.

8.3.3 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

• Receiver failsafe

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SN65HVD30, SN65HVD31, SN65HVD32

SN65HVD33, SN65HVD34, SN65HVD35

SLLS665M – SEPTEMBER 2005 – REVISED FEBRUARY 2023

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