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 mix 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’ (~1200m), so the wide CMR is necessary to

handle ground potential differences, as well as voltages

induced in the cable by external fields.

Receiver Features

These devices utilize a differential input receiver for maximum

noise immunity and common mode rejection. Input sensitivity

is

±200mV, as required by the RS422 and RS-485

specifications. 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, or

industrial environments, where induced voltages are a

realistic concern.

Receiver input resistance surpasses the RS-422 spec of

4k

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

12k

Ω minimum. The 20kΩ input resistance allows at least 50

devices on the RS-485 bus. All the receivers include a “fail-

safe if open” function that guarantees a high level receiver

output if the receiver inputs are unconnected (floating).

Receivers easily meet the data rate supported by the driver,

and receiver outputs are three-statable via the active low RE

input.

Driver Features

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

delivers at least 2.3V across a 54

Ω load (RS-485/

PROFIBUS), and at least 2.5V across a 100

Ω load (RS-422)

delay skew to maximize bit width, and to minimize EMI.

Outputs of the drivers are not slew rate limited, so faster

output transition times allow data rates of at least 40Mbps

with the ISL4486, and 30Mbps with the ISL81486. Driver

outputs are three-statable via the active high DE input.

For parallel applications, bit-to-bit skews between any two

ISL4486 transmitter and receiver pairs are guaranteed to be

no worse than 15ns (6ns max for any two Tx, 9ns max for

any two Rx).

Data Rate, Cables, and Terminations

Twisted pair is the cable of choice for RS-485, RS-422, and

PROFIBUS 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/RS-422 are intended for network lengths up to 4000’

(~1200m), but the maximum system data rate decreases as

the transmission length increases. According to guidelines in

the RS-422 and PROFIBUS specifications, networks

operating at data rates in excess of 3Mbps should be limited

to cable lengths of 100m (328 feet) or less, and the

PROFIBUS specification recommends that the more

expensive “Type A” (22AWG) cable be used. Nevertheless,

the ISL4486’s large differential output swing, fast transition

times, and high drive-current output stages allow operation

at 40Mbps over standard “CAT5” cables in excess of 400

feet (121m). Figure 8 details the ISL4486 performance at

this condition, with a 120

Ω termination resistor at both the

driver and the receiver ends. Note that the differential signal

delivered to the receiver at the end of the cable (A-B) still

exceeds 1V, so even longer cables could be driven if lower

noise margins are acceptable. If more noise margin is

desired, shorter cables produce a larger receiver input signal

as illustrated in Figure 7. Performance should be even better

if the “Type A” cable is utilized. The lower data rate of the

ISL81486 allows for driving longer cables.

To minimize reflections, proper termination is imperative when

using these high data rate transceivers. 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 for “CAT5”, and 220Ω for “Type A”) 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. Multipoint (multi-driver) systems require that

the main cable be terminated in its characteristic impedance

at both ends. 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. These

transmitters meet 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. In the event of a major short circuit

condition, the devices also include 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 temperature drops about

15 degrees. If the contention persists, the thermal

shutdown/reenable cycle repeats until the fault is cleared.

Receivers stay operational during thermal shutdown.

ISL4486, ISL81486