SN65HVD30-EP, SN65HVD31-EP, SN65HVD32-EP

SN65HVD33-EP, SN65HVD34-EP, SN65HVD35-EP

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SGLS367E – SEPTEMBER 2006 – REVISED SEPTEMBER 2015

Typical Application (continued)

10.2.1.2 Stub Length

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

the stub, must be as short as possible. Stubs present a nonterminated piece of bus line that can introduce

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

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

shown in Equation 1.

where:

•

•

c is the speed of light (3 × 10

•

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

(1)

Per Equation 1, Table 8 shows the maximum cable-stub lengths for the minimum driver output rise times of the

SN65HVD3x-EP full-duplex family of transceivers for a signal velocity of 78%.

Table 8. Maximum Stub Length

MAXIMUM STUB LENGTH

MINIMUM DRIVER OUTPUT RISE TIME

DEVICE

(ns)

(m)

(ft)

SN65HVD30

4

0.1

0.3

SN65HVD31

25

0.6

1.9

SN65HVD32

120

2.8

9.2

SN65HVD33

4

0.1

0.3

SN65HVD34

25

0.6

1.9

SN65HVD35

120

2.8

9.2

10.2.1.3 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 SN65HVD30 and SN65HVD33 devices

are 1/2 UL transceivers, it is possible to connect up to 64 receivers to the bus. Likewise, the SN65HVD31,

SN65HVD32, SN65HVD34, and SN65HVD35 devices are 1/8 UL transceivers that can support up to 256

receivers.

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