8

Driver Features

The RS-485, RS-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) even with

maximize bit width, and to minimize EMI, and it is tri-statable

via the active high DE input.

Outputs of the ISL43485 driver are not slew rate limited, so

faster output transition times allow data rates of at least

30Mbps.

Data Rate, Cables, and Terminations

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 this IC.

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

are often limited to lengths of less than one hundred feet.

Figure 6 details the ISL43485’s 30Mbps performance driving

200’ of “CAT5” cable terminated in 120

Ω at both ends. Note

that the differential signal delivered to the receiver at the end

of the cable (A-B) still exceeds 1.5V peak. Longer cable

lengths are possible by reducing the data rate, as shown in

Figure 7 for a data rate of 20Mbps.

To minimize reflections, proper termination is imperative

when using this 30Mbps 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. 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

ISL43485 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, it utilizes 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

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.

Low Power Shutdown Mode

This BiCMOS transceiver uses a fraction of the power

required by its bipolar counterparts, nevertheless, the

ISL43485 includes a shutdown feature that reduces the

shutdown whenever the receiver and driver are

period of at least 300ns. Disabling both the driver and the

receiver for less than 80ns guarantees that shutdown is not

entered.

Note that receiver and driver enable times increase when

these devices enable from shutdown. Refer to Notes 5-9, at

the end of the Electrical Specification table, for more

information.

ISL43485