Many applications need both an RS-232 and RS-485 serial port. The dual-protocol devices integrate both transceivers into a single package. The single-port device can be programmed for either interface. The two-port configuration is very flexible as each port can be either protocol.

Both 3.3V and 5V supply options are available in SOIC and QFN packages. The QFN package has additional features including a logic supply pin (VL) and selectable RS-485 data rates. The RS-232 data rate can operate up to 400kbps.

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ISL3333xE/5xE Dual Protocol RS-485/RS-232 Transceivers Webinar

This webinar focuses on the ISL33334E, ISL33337E, ISL33354E and ISL33357E dual protocol transceivers for RS-232 and RS-485 interface applications. Included is a short overview of RS-232 and RS-485 interface features, education on what a dual protocol transceiver is, dual protocol transceiver features and applications and a list of Intersil's dual protocol transceiver families.


ISL3333xE/ISL3335xE Dual Protocol (RS-232/RS-485) Transceivers

Today's session focuses on the dual protocol transceivers for RS-232 and RS-485 interface applications, the ISL33334E, isl33337e, isl33354e and isl33357e.

This training module gives a short overview of RS-232 and RS-485 interface features, explains what a dual protocol transceiver is, goes over its features and applications and lists our families of dual protocol transceivers.

Interface Products from Intersil

Intersil was an original innovator during the early evolution of serial interface standards, including the introduction of the charge pump eliminating the negative supply voltage requirement. Our extensive portfolio gives our customers all of the choices they need to meet their serial communication needs.


Applications of fixed-port dual protocol transceivers include gaming applications (e.g. slot machines), industrial PCs, factory automation, security networks, industrial process control networks and point of sale (POS) equipment.

In almost all of these applications, data for operation control, equipment configuration and sensory inputs are transferred via RS-232. In order to exchange larger amount of data between multiple pieces of equipment and a central host controller, a multipoint RS-485 network is utilized.

Single Protocol Signal Overview: RS-232

Dual protocol transceivers support the two protocols: RS-232 and RS-485.

RS-232 is a single-ended interface requiring a ground wire between signal source and sink. The interface uses two conductors for transmitting and receiving data, TxD and RxD, respectively. Because RS-232 drivers have no tristate capability, the interface can be used for point-to-point links only.

Bus voltages can range from +3V to +15V for a logic low, and from -3V to -15V for a logic high, with reference to signal ground. RS-232 operates in full-duplex mode, meaning data can be transmitted and received at the same time.

The standard specifies a maximum slew rate of SR = 30V/µs, which at large voltage swings limits the data rate to 20kbps, while at low voltage swings the data rate can be increased to 1Mbps.

RS-232 is often used to exchange control and configuration data between a host controller and associated equipment. Here, the maximum distance of a data link is limited to around 20m (60ft).

Single Protocol Signal Overview: RS-485

RS-485 is a differential requiring no ground wire between signal source and sink. The interface uses two conductors for transmitting and receiving data, A and B. Because RS-485 drivers can be tristated, the interface can be used for multipoint data links.

The differential bus voltage, Vod = Va - Vb, must be ≥ ±1.5V. Positive Vod represents a logic high, negative Vod a logic low. While the bus voltage can ride on large common-mode voltages ranging from Vcm = -7V to +12V, the common-mode component is rejected by the differential RS-485 receiver.

RS-485 networks can be designed for full- or half-duplex operation. Full-duplex requires a conductor pair for transmit and another one for receive data. Half-duplex is bidirectional, using only two conductors, but limiting data traffic to one direction at a time: transmit or receive.

Maximum data rates range from 100kbps @ 1200m (4000ft) up to 100Mbps @ 10m (30ft) cable length. Because of its high reliability in electrical noisy environments, RS-485 has become the interface work horse of the industry.

Solution: Configurable Dual Protocol Transceivers

Configurable dual protocol transceivers enable the configuration of an interface port from RS-232 to RS-485 mode via means of a select pin, SEL. A low on SEL configures a port for RS-232, a high for RS-485 operation.

One interface port provides one differential RS-485 transceiver, or two single-ended RS-232 transceivers.

There are one and two port, dual protocol transceivers. In a two-port device, each port has its dedicated select pin, thus allowing the configuration of:

  • 4 x RS-232 transceivers
  • 2 x RS-232 and
  • 1 x RS-485 transceivers
  • 2 x RS-485 transceivers

ISL3333x4E and ISL333x7E Overview

Initially, fully configurable transceivers presented king-pin devices in industrial networking. Nowadays however, customers prefer simpler devices at lower cost. Hence, fixed-port devices that still support dual protocols, but also enable operation down to 3.15V are available in space-saving packages such as our 6mm x 6mm QFN.

Members of the fixed dual port transceivers family are the ISL33334, ISL33354, ISL33337, and ISL33357. Note that the fourth digit represents the nominal supply voltage (3 = 3.3V, 5 = 5.0V), while the fifth digit indicates the transceiver package (4 = SSOP-28, 7 = QFN-40).

Fixed Dual Protocol Transceivers

Features of the fixed dual protocol transceivers are:

  • A highly efficient charge pump allowing the use of space saving 0.1µF capacitors, while enabling device operation down to 3.15V Vcc.
  • Robust on-chip ESD structures protecting all bus pins against up to 15kV electrostatic discharges per HBM specified in JEDEC standard JESD22-A114F
  • Two fast RS-232 transceivers with typical data rates of 400kbps
  • One high-speed RS-485 receiver with up to 20Mbps maximum data rate and failsafe operation during open- and short-circuited bus lines
  • One speed selectable RS-485 driver with high different output voltage of Vod = 2.7V (@5V Vcc) for large noise margin, and maximum data rates of 155kbps in slow mode and 20Mbps in fast mode
  • Low, 40μA supply current in shutdown mode during which all drivers, receivers and the charge pump are disabled
  • Note 1: Most competition devices maintain charge pump operation during shutdown, thus drawing significantly higher supply currents
  • True flow-through design with bus pins on the right and logic pins on the left side of the chip drastically ease PCB layout
  • Note 2: Many competition devices have bus and logic pins on both sides of the chip

Application Example: Industrial Network

This application stresses the versatility of dual protocol transceivers.

When implemented into industrial PCs (IPC) or programmable logic controllers (PLC), dual protocol transceivers can provide direct links to instruments and machines equipped with RS-232 or RS-485 interfaces.

If however, networking of multiple equipment is required, dual protocol transceivers can be configured to level translators or interface converters.

The following slides show three variants of level translators using the ISL33334, each serving a different interface purpose.

RS-232 to RS-485 Converter Configuration #1

Configuration 1 adds long distance drive capability to an RS-232 channel by connecting the logic I/O of the RS-232 transceiver with the logic I/O of the RS-485 transceiver. The full-duplex structure of the RS-232 interface is maintained for the entire data link. However, since the enable pin of the RS-485 driver is fixed to Vcc potential, the driver remains active all the time. This type of level translator is therefore only suited for point-to-point data links.

RS-232 to RS-485 Converter Configuration #2

Configuration 2 is similar to Configuration 1 with the exception that a second RS-232 channel (receiver) is used to turn the RS-485 driver on and off. This allows the level translator to be used in full-duplex multipoint networks. Since the input of the RS-485 receiver is high-impedance, the receiver can remain constantly active.

RS-232 to RS-485 Converter Configuration #3

Configuration 3 has the RS-485 transceiver configured for half-duplex operation, which requires only two conductors for long-haul networks. Note that for long distance transmission lines the RS-485 transceiver should be set to slow-mode (115kbps).

The RS-232 receiver controls the operation mode of the RS-485 transceiver. A low at R1OUT (and hence at DI) sets the RS-485 transceiver into transmit mode by enabling the driver and disabling the receiver. A low at DI is therefore actively converted into a negative bus voltage. A high at R1OUT accomplishes the opposite and switches the RS-485 transceiver into receive mode. With the driver outputs now being high-impedance, a positive bus voltage is established by the failsafe biasing resistor network.

The small deglitch circuit consisting of an inverter and a R-C-D unit prevents the occurrence of glitches on the bus during the transitions from drive into receive mode.

This half-duplex level translator is the most efficient of all, as it saves an entire conductor pair versus a full-duplex level translator, and also frees up the second RS-232 channel, which can now be used for other interface purposes.

Dual Protocol Portfolio (Fixed and Configurable)

View part comparison table

This slide lists Intersil’s portfolio of single and two-port, dual protocol transceivers.

The first four rows in blue depict the recently released, fixed-port transceivers.

As can be seen, the versatility of the fixed-port devices has been significantly reduced by:

  • Reducing the choice of RS-485 data rates from 3 to 2
  • Keeping the RS-232 data rate about the same at 400kbps
  • Removing the logic supply pin, VLOGIC
  • Maintaining the same polarity of the RS-485 receiver enable pin across all devices

Available Tools and Support

There are two datasheets available for these dual protocol transceivers and a white paper that discusses how these transceivers ease the design of industrial interfaces. Complete information on the dual protocol RS-232 and RS-485 transceivers can be found on our website.


Intersil is a major supplier of serial interface devices. To summarize, we've been offering serial interface products for nearly 40 years. These fixed-channel devices offer a lower cost alternative to our programmable dual protocol offering and present a great opportunity to eliminate one BOM component at a competitive price.