7

Application Information

RS-485 and RS-422 are differential (balanced) data

transmission standards for use in long haul or noisy

environments. RS-422 is a subset of RS-485, so RS-485

transceivers are also RS-422 compliant. RS-422 is a point-

to-multipoint (multidrop) standard, which allows only one

driver and up to 10 (assuming one unit load devices)

receivers on each bus. RS-485 is a true multipoint standard,

which allows up to 32 one unit load devices (any

combination of drivers and receivers) on each bus. To allow

for multipoint operation, the RS-485 spec requires that

drivers must handle bus contention without sustaining any

damage.

Another important advantage of RS-485 is the extended

common mode range (CMR), which specifies that the driver

outputs and receiver inputs withstand signals that range

from +12V to -7V. RS-422 and RS-485 are intended for runs

as long as 4000’, so the wide CMR is necessary to handle

ground potential differences, as well as voltages induced in

the cable by external fields.

Receiver Features

The ISL4485E utilizes a differential input receiver for

maximum noise immunity and common mode rejection. Input

sensitivity is

±200mV, as required by the RS-422 and RS-485

specifications.

Receiver input impedance surpasses the RS-422 spec of

4k

Ω, and meets the RS-485 “Unit Load” requirement of

12k

Ω minimum.

Receiver inputs function with common mode voltages as

great as

±7V outside the power supplies (i.e., +12V and

-7V), making them ideal for long networks where induced

voltages are a realistic concern.

The receiver includes a “fail-safe if open” function that

guarantees a high level receiver output if the receiver inputs

are unconnected (floating). The output is three-statable via

the active low RE input, and the receiver easily meets the

20Mbps data rate.

Driver Features

The RS-485/422 driver is a differential output device that

delivers at least 1.5V across a 54

Ω load (RS-485), and at

least 2V across a 100

Ω load (RS-422). The ISL4485E driver

features low propagation delay skew to maximize bit width,

and to minimize EMI, and the outputs are three-statable via

the active high DE input.

Outputs of ISL4485E drivers are not slew rate limited, so

faster output transition times allow data rates up to 20Mbps.

Data Rate, Cables, and Terminations

Twisted pair is the cable of choice for RS-485/422 networks.

Twisted pair cables tend to pick up noise and other

electromagnetically induced voltages as common mode

signals, which are effectively rejected by the differential

receivers in these ICs.

RS-485/422 are intended for network lengths up to 4000',

but the maximum transmission length decreases as the data

rate increases. According to guidelines in the RS-422

specification, a 20Mbps network should be limited to less

than 50' of 24 AWG twisted pair. Nevertheless, the

ISL4485E's large differential voltage swing, fast transition

times, and high drive-current output stages allow operation

at 20Mbps in RS-485/422 networks as long as 450'. Figure 7

details ISL4485E operation at 20Mbps driving 300' of CAT 5

cable terminated in 120

Ω at the driver and the receiver (i.e.,

double terminated). The acceptance criteria for this test was

the ability of the driver to deliver a 1.5V differential signal to

the receiver at the end of the cable (i.e., |A-B|

≥ 1.5V). If a

more liberal acceptance criteria is used, the distance can be

further extended. For example, Figure 8 illustrates the

performance in the same configuration, but with a cable

length of 450', and an acceptance criteria of no more than

6dB attenuation across the cable (i.e., |A-B| = |Y-Z|/2).

Driver differential output voltage decreases with increasing

differential load capacitance, so maintaining a 1.5V

differential output requires a data rate reduction, as shown in

Figure 9.

To minimize reflections, proper termination is imperative

when using this 20Mbps device. In point-to-point, or point-to-

multipoint (single driver on bus) networks, the main cable

should be terminated in its characteristic impedance

(typically 120

Ω) at the end farthest from the driver. In multi-

receiver applications, stubs connecting receivers to the main

cable should be kept as short as possible (preferably less

than 12 inches). Multipoint (multi-driver) systems require that

the main cable be terminated in its characteristic impedance

at both ends. Again, stubs connecting a transceiver to the

main cable should be kept as short as possible.

Built-In Driver Overload Protection

As stated previously, the RS-485 spec requires that drivers

survive worst case bus contentions undamaged. The

ISL4485E device meets this requirement via driver output

short circuit current limits, and on-chip thermal shutdown

circuitry.

The driver output stages incorporate short circuit current

limiting circuitry which ensures that the output current never

exceeds the RS-485 spec, even at the common mode

voltage range extremes. Additionally, these devices utilize a

foldback circuit which reduces the short circuit current, and

thus the power dissipation, whenever the contending voltage

exceeds either supply.

In the event of a major short circuit condition, this device

also includes a thermal shutdown feature that disables the

drivers whenever the die temperature becomes excessive.

This eliminates the power dissipation, allowing the die to

cool. The drivers automatically reenable after the die

ISL4485E