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THVD1500

SLLSEY4A – JULY 2017 – REVISED NOVEMBER 2018

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Typical Application (continued)

9.2.1.2 Stub Length

When connecting a node to the bus, the distance between the transceiver inputs and the cable trunk, known as

the stub, should be as short as possible. Stubs present a non-terminated piece of bus line which can introduce

reflections as the length of the stub increases. As a general guideline, the electrical length, or round-trip delay, of

a stub should be less than one-tenth of the rise time of the driver, thus giving a maximum physical stub length as

shown in Equation 1.

L(STUB) ≤ 0.1 × tr × v × c

where

•

tr is the 10/90 rise time of the driver

•

c is the speed of light (3 × 10

8 m/s)

•

v is the signal velocity of the cable or trace as a factor of c

(1)

9.2.1.3 Bus Loading

The RS-485 standard specifies that a compliant driver must be able to driver 32 unit loads (UL), where 1 unit

load represents a load impedance of approximately 12 k

Ω. Because the THVD1500 consists of 1/8 UL

transceivers, connecting up to 256 receivers to the bus is possible.

9.2.1.4 Receiver Failsafe

The differential receivers of the THVD1500 are failsafe to invalid bus states caused by the following:

•

Open bus conditions, such as a disconnected connector

•

Shorted bus conditions, such as cable damage shorting the twisted-pair together

•

Idle bus conditions that occur when no driver on the bus is actively driving

In any of these cases, the differential receiver will output a failsafe logic high state so that the output of the

receiver is not indeterminate.

Receiver failsafe is accomplished by offsetting the receiver thresholds such that the input indeterminate range

does not include zero volts differential. In order to comply with the RS-422 and RS-485 standards, the receiver

output must output a high when the differential input VID is more positive than 200 mV, and must output a low

when VID is more negative than –200 mV. The receiver parameters which determine the failsafe performance are

VIT+, VIT–, and VHYS (the separation between VIT+ and VIT–). As shown in the Electrical Characteristics table,

differential signals more negative than –200 mV will always cause a low receiver output, and differential signals

more positive than 200 mV will always cause a high receiver output.

When the differential input signal is close to zero, it is still above the VIT+ threshold, and the receiver output will

be high. Only when the differential input is more than VHYS below VIT+ will the receiver output transition to a low

state. Therefore, the noise immunity of the receiver inputs during a bus fault conditions includes the receiver

hysteresis value, VHYS, as well as the value of VIT+.