Driver Input

Driver Output

Receiver Input

Receiver Output

ISO15, ISO35, ISO15M, ISO35M

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SLOS580G – MAY 2008 – REVISED MARCH 2015

Typical Application (continued)

10.2.1 Design Requirements

RS-485 is a robust electrical standard suitable for long-distance networking that may be used in a wide range of

applications with varying requirements, such as distance, data rate, and number of nodes.

Table 3. Design Parameters

PARAMETER

VALUE

Pullup and Pulldown Resistors

1 k

Ω to 10 kΩ

Decoupling Capacitors

100 nF

10.2.2 Detailed Design Procedure

There is an inverse relationship between data rate and cable length, which means the higher the data rate, the

shorter the cable length; and conversely, the lower the data rate, the longer the cable 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 nonterminated 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. 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 these devices consists of 1/8 UL transceivers, connecting up to 256 receivers to the bus is possible.

10.2.3 Application Curve

Figure 20. Typical Input and Output Waveforms

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