7

Driver Features

The RS-485, RS-422 driver is a differential output device

that delivers at least 2V across a 54

Ω load (RS-485), and at

least 2.5V across a 100

Ω load (RS-422) even with

to maximize bit width, and to minimize EMI.

The driver is three-statable via the active high DE input.

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

output transition times allow data rates of at least 30Mbps.

Data Rate, Cables, and Terminations

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

4000’, but the maximum system data rate decreases as the

transmission length increases. Devices operating at 30Mbps

usually are limited to lengths of a couple hundred feet, but

the larger differential output voltage of this transceiver allows

that distance to be pushed past 350’. Figure 7 illustrates the

30Mbps performance of the ISL81485 driving 350’ of CAT5

cable, terminated in 120

Ω at both the driver and receiver

ends. As shown, the differential signal (A-B) delivered to the

receiver inputs at the end of the cable is still greater than

1.5V (i.e., 7.5 times the required Rx sensitivity). Thus, even

longer cables can be driven if lower noise margins are

acceptable.

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

Proper termination is imperative, when using a 30Mbps

device, to minimize reflections. 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. 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. The

ISL81485 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.

In the event of a major short circuit condition, the 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 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.

FIGURE 7. DRIVER AND RECEIVER WAVEFORMS DRIVING

350 FEET (107 METERS) OF CAT5 CABLE

(DOUBLE TERMINATED WITH 120

Ω)

FIGURE 8. DRIVER OUTPUT CURRENT vs SHORT CIRCUIT

VOLTAGE

TIME (20ns/DIV)

-3

1.5

3

-1.5

0

0

5

0

5

DI = 30Mbps

RO

DRIVER+CABLE DELAY

(~490ns)

A - B

OUTPUT VOLTAGE (V)

-7 -6

-4

-2

0

2

4

6

8

10

12

-60

-40

-20

0

20

40

60

80

100

120

140

160

-80

-100

-120

Y OR Z = HIGH

-140

-160

Y OR Z = LOW

Y OR Z = HIGH

ISL81485