ISO1430DWR datasheet(31/49 Pages) TI2

31 / 49 page

10.2.2.1 Data Rate and Bus Length

The RS-485 standard has typical curves similar to those shown in Figure 10-4. These curves show the inverse

relationship between signaling rate and cable length. If the data rate of the payload between two nodes is lower,

the cable length between the nodes can be longer.

Data Rate (bps)

10000

1000

100

10 k

100 k

10 M

100 M

Conservative

Characteristics

5%, 10%, and 20% Jitter

Figure 10-4. Cable Length vs Data Rate Characteristics

Use Figure 10-4 as a guideline for cable selection, data rate, cable length and subsequent jitter budgeting.

10.2.2.2 Stub Length

In an RS-485 network, the distance between the transceiver inputs and the cable trunk is known as the stub. The

stub should be as short as possible when a node is connected to the bus. Stubs are a non-terminated piece of

bus line that can introduce reflections of varying phase as the length of the stub increases. The electrical length,

or round-trip delay, of a stub should be less than one-tenth of the rise time of the driver as a general guideline.

Therefore, the maximum physical stub length (L(STUB)) is calculated as shown in Equation 3.

L(STUB) ≤ 0.1 × tr × v × c

(3)

where

• tr is the 10/90 rise time of the driver.

• c is the speed of light (3 × 108 m/s).

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

10.2.2.3 Bus Loading

The current supplied by the driver must supply into a load because the output of the driver depends on

this current. Add transceivers to the bus to increase the total bus loading. The RS-485 standard specifies a

hypothetical term of a unit load (UL) to estimate the maximum number of possible bus loads. The UL represents

a load impedance of approximately 12 kΩ. Standard-compliant drivers must be able to drive 32 of these ULs.

The ISO14xx devices have 1/8 UL impedance transceiver and can connect up to 256 nodes to the bus.

10.2.3 Application Curves

Below eye diagram of ISO145x device indicates low jitter and wide open eye at maximum data rate of 50 Mbps.

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ISO1450, ISO1452, ISO1410, ISO1412, ISO1430, ISO1432

SLLSF22H – APRIL 2018 – REVISED JUNE 2024

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