ISL3159E

11

FN6364.2

August 25, 2015

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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 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.

The receiver input resistance of 50kΩ surpasses the RS-422

spec of 4kΩ, and is five times the RS-485 “Unit Load” (UL)

requirement of 12kΩ minimum. Thus, the ISL3159E 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) at

networks).

Receivers easily meet the 40Mbps 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 54Ω load (RS-485/

PROFIBUS), and at least 2.6V across a 100Ω 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.

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

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

ISL3159E 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 ISL3159E 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

ISL3159E in “star” and other multiterminated, “nonstandard”

network topologies. Figure 9 on page 8, details the

lines for four (30Ω) and six (20Ω) 120Ω terminations. Figure 9

shows that the driver typically delivers 1.9/1.5V into 4/6

typically delivers RS-485 voltage levels with 2x to 3x the

number of terminations.

ESD Protection

All pins on the ISL3159E 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.

AIR-GAP DISCHARGE TEST METHOD

For this test method, a charged probe tip moves toward the IC

pin until the voltage arcs to it. The current waveform delivered to

the IC pin depends on approach speed, humidity, temperature,

etc., so it is more difficult to obtain repeatable results. The

ISL3159E RS-485 pins withstand ±15kV air-gap discharges.