9

FN6587.1

February 18, 2008

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 (Rx) Features

This transceiver 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 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.

The receiver input resistance of 50k

Ω surpasses the RS-422

spec of 4k

Ω, and is 5x the RS-485 “Unit Load” (UL)

requirement of 12k

Ω minimum. Thus, the ISL3259E is

known as a “one-fifth UL” transceiver, and there can be up to

160 devices on the RS-485 bus while still complying with the

RS-485 loading specification.

The receiver is a “full fail-safe” version that guarantees a

high level receiver output if the receiver inputs are

unconnected (floating), shorted together, or connected to a

terminated bus with all the transmitters disabled

(terminated/undriven).

Rx outputs deliver large low state currents (typically >30mA)

networks.

Receivers easily meet the 100Mbps data rate supported by

the driver, and the receiver output is tri-statable via the active

low RE input.

Driver (Tx) Features

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

delivers at least 2.1V across a 54

Ω load (RS-485/

PROFIBUS), and at least 2.6V 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 up to100Mbps. Driver

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

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

ISL3259E transmitter and receiver pairs are guaranteed to

be no worse than 8ns (4ns max for any two Tx, 4ns max for

any two Rx).

The ISL3259E driver design delivers larger differential output

most RS-485 transmitters can deliver. The minimum ±2.1V

than two bus terminations, which allows for utilizing the

ISL3259E in “star” and other multi-terminated, “non-standard”

six (20

Ω) 120Ω terminations. The figure shows that the driver

terminations, but the ISL3259E typically delivers RS-485

voltage levels with 2x to 3x the number of terminations.

ESD Protection

All pins on the ISL3259E include class 3 (>9kV) Human

Body Model (HBM) ESD protection structures, but the

RS-485 pins (driver outputs and receiver inputs)

incorporate advanced structures allowing them to survive

ESD events in excess of ±16.5kV HBM and ±15kV

IEC61000-4-2. The RS-485 pins are particularly vulnerable

to ESD strikes because they typically connect to an

exposed port on the exterior of the finished product. Simply

touching the port pins, or connecting a cable, can cause an

ESD event that might destroy unprotected ICs. These new

ESD structures protect the device whether or not it is

powered up, and without degrading the RS-485 common

mode range of -7V to +12V. This built-in ESD protection

eliminates the need for board level protection structures

(e.g., transient suppression diodes), and the associated,

undesirable capacitive load they present.

IEC61000-4-2 Testing

The IEC61000 test method applies to finished equipment,

rather than to an individual IC. Therefore, the pins most likely

to suffer an ESD event are those that are exposed to the

outside world (the RS-485 pins in this case), and the IC is

tested in its typical application configuration (power applied)

rather than testing each pin-to-pin combination. The

IEC61000 standard’s lower current limiting resistor coupled

with the larger charge storage capacitor yields a test that is

much more severe than the HBM test. The extra ESD

protection built into this device’s RS-485 pins allows the

design of equipment meeting level 4 criteria without the need

for additional board level protection on the RS-485 port.

ISL3259E