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ISL81387IAZ: A Dual Protocol Transceiver with ESD Protection

ISL81387IAZ is a dual protocol transceiver, ideal for configuring devices based on the RS-232 and RS-485 standards.

This blog post will help you understand how the dual protocol works.

User-Based Configurations

ISL81387IAZ is a user-configurable BiCMOS interface Integrated Circuit (IC). It can be configured to fit into a variety of performances, including the RS-232 transceiver, RS-422 and the RS-485 differential transceiver.

Power Management

Balancing the performance and power usage of a transceiver are two important considerations. We are pleased to see that ISL81387IAZ has some measures in place to help this function better.

For example, the low-current shutdown mode is an excellent solution, as it can turn off the device once it goes below 35µA.

Board Layout Simplicity


ISL81387IAZ also has a dedicated feature called true flow-through pinouts. These pinouts help the transceiver in offering the best mapping or layout for configurations.

ISL81387IAZ also supports the loopback mode; a function that allows for the self-testing of the circuit board functions.

The loopback mode also helps in keeping the transceiver active. For example, it comes in handy to keep the outputs connected to the loads during the loopback function. That way, connection-related issues, such as shorted cables or connections can be easily detected during the operation.

Problem-Free Communications

By “problem-free communications,” we refer to the data transfer rate. ISL81387IAZ’s data transfer rate is between 115kbps and 20 Mbps. Both of these are user-programmable and comply with the RS-485 transmitter mode.

The transmitter supports up to three (3) data rates, which are both slew rate limited.

Open-Short Function

Also called the full fail-safe operation, the open-short function is one feature that keeps this transceiver active. The activeness is culled from the ability to keep the Rx output in a high state, especially if the inputs are shorted or opened together.

The RS-485 Mode

ISL81387IAZ can be configured through any of the modes, as it is a dual protocol transceiver. The RS-485 mode is one of such modes and it offers additional features to the transceiver. Here are some of these features:

Charge Pump Disabling

When the ISL81387IAZ is in the RS-485 mode, it often disables the charge pump. For emphasis, the charge pump is often activated during the RS-232 mode usage, as it aids the generation of the compliant ±5 Tx output levels.

However, once the RS-485 mode is active, the charge pump’s capacitors is eliminated, likewise the charge pump is disabled.

Low Noise

The disabling of the charge pump also leads to lesser noise during the transceiver’s operations. Besides, ISL81387IAZ has a general noise-limiting capability, via the large (2.7-volt) differential VOUT. This function can improve the noise immunity capabilities of the RS-422 and RS-485 networks/modes.


Since the charge pump is active, it tends to consume more power. By disabling and eliminating the further usage, the RS-485 mode saves more power in the process.

Packaging as a Performance Catalyst

ISL81387IAZ uses a QFN package/case – and that serves as a boost to the performance. These are some of the catalyst-like features:

1. Pin Flexibility

The QFN package/case uses a combination of the active high and the low Rx outputs. This function enables the pins to have an increased flexibility; up to the point that the combined function of the DEN and RXEN functions allow the Tx/Rx directional control via a single signal.

2. Lower Speed

The QFN package supports a lower speed and an edge rate option, capable of clocking 115 kilobytes per second. The combined function of the edge rate option and the lower speed are ideal for making ISL81387IAZ ideal for longer bus lengths or optimization for EMI-sensitive designs.

3. Supply Voltage Compatibility

As a way of making the supply voltage compatible, the QFN package has been optimized along the lines of logic supply voltage pin (VL). The pin makes the package ideal for enabling the VOH level of logic outputs. By enabling this, it becomes easier to combine the switching points of the logic inputs compatible with the supply voltage in another mixed voltage system.

Product Attributes

Here is a tabulated breakdown of all of the components making up ISL81387IAZ’s dual protocol transceiver:

Operating TemperatureBetween -40˚C and 85˚C
Voltage – Supply (minimum to maximum)Between 4.5 volts and 5.5 volts
Supported ProtocolsRS-485, RS-232 and RS-422
Mounting StyleSurface Mount Technology (SMT)
Data (transfer) Rate20Mbps
Number of Supported Receivers and Drivers2/2
Receiver Hysteresis35 mV

Important Points to Note about ISL81387IAZ’s Dual Protocol Architecture

Transceivers are dedicated devices that necessary hardware resources to enable the real-time communication of devices over an extended cable length or trace length.

4. The Need to Find a Balance in Transceiver Performance

The major industrial interfaces used in ISL81387IAZ are the RS-232 and the RS-485 standards. The two offer improved functions and that is why they have been combined inside the ISL81387IAZ transceiver.

On the one hand, we have the RS-232, a transceiver interface standard that became popular in the early 60s. It works by providing a single-ended, full-duplex interface to enable point-based consumer electronics communication. The RS-232 interface standard also combines receiver grounds and ground wire connection to facilitate a common interface for receiving and transmitting (data) signals.

On the other hand, we have the RS-485 interface standard, which became popular in the early 1980s. The most outstanding function is the drastic improvements made in the data transmission, across longer distances – and even in noisy environments and working conditions.

Those are the basics. Why were the RS-232 and RS-485 interface standards combined? The formation of a dual protocol interface with the two is because of the need to combine the two interfaces’ functions, as both of them have viable functions to offer.

Through this combined function, the interfaces now offer individualized solutions. For example, the RS-232 interface standard helps in the following ways:

  • Enabling interface connection to either the single network or extended over long distances.
  • It enables an easy routing of signal traces to the local controllers.
  • The RS-232 interface standard also boosts the data rates, up to 400kbps. It does it excellently, without exceeding or going above the recommended, maximum slew rates.

On the other hand, the integration of the RS-485 interface standards makes the following contributions to the ISL81387IAZ dual protocol transceiver:

  • Implementation of a high-speed mode.
  • It supports the networking of up to 32 unit loads via a multi-point bus topology.

Wrapping Up

ISL81387IAZ combines the RS-232 and the RS-485 interface standards to make the transceiver flexible and versatile. The versatility is derived from the local controller’s capability of acting independently as an interface converter or driving the different bus systems.

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