LOW VOLTAGE, SINGLE SUPPLY, 232-STANDARD INTERFACE SOLUTIONS

ABSTRACT

Features and key specifications for Texas Instruments (TI^TM) new low‑voltage, single‑supply, 232‑standard‑compliant interface devices are presented, as well as test data comparing TI versus competitor same‑function devices, Evolution of the 232 standard is discussed relative to TI devices that have significantly greater capability than the 232 standard requires.

TABLE OF CONTENTS

Title Page                                                                                   

Certification                                                                               i

Dedication                                                                                  ii

Acknowledgement                                                                     iii

Abstract                                                                                     iv

Table of Contents                                                                       v

Chapter One                                                               

Introduction                                                                               1

Brief Overview of the 232 Standard                                         2

Technology Evolution                                                                3

ANSI TIA/EIA 232-F Electrical Specification                           6

232-Standard Handshaking Signals for DB9S Connections       8

Chapter Two

Linear Power Supply                                                                 10

Switching vs Linear Power Supplies                                         13

Pulse Width Modulation                                                           14

Off-Line Switching Supply                                                         21

Forward Converter                                                                    22

Chapter Three

Symmetrical Converters                                                           24

Push-Pull Converter                                                                 24

Life Support Policy                                                                    27

Chapter Four

Typical Design Applications                                                      29

Analysis of IT Competitor Equivalent Devices                         36

CHAPTER ONE

INTRODUCTION       

This application provides information about the new low‑voltage, single‑supply interface devices produced by TI. These devices include the popular three driver/five receiver data terminal equipment (DTE) device named MAX3243, and specifically are designed for the personal computer (PC) serial port 1/0, which usually is used with the 9‑pin D139S connector. Another new device is the five‑d river/three‑receiver data‑circuit‑terminating equipment (DCE) device named MAX3238, which is the complement to the DTE device, and is commonly used on the peripheral end of the connecting cable. Also, the MAX3221, which is the smallest possible 232‑standard solution, with one driver and one receiver per package, is another member of this family of devices.

Virtually every piece of electronic equipment e.g., comput­ers and their peripherals, calculators, TV and hi‑fi equip­ment, and instruments, is powered from a DC power source, be it a battery or a DC power supply. Most of this equipment requires not only DC voltage but voltage that ‑is also well filtered and regulated. Since power supplies are so widely used in electronic equipment, these devices now comprise a worldwide segment of the electronics market in excess of $5 billion annually.

There are three types of electronic power conversion de­vices in use today which are classified as follows according to their input and output voltages: 1) DC/DC converter; 2) the AC/DC power supply; 3) the DC/AC inverter. Each has its own area of use but this paper will only deal with the first two, which are the most commonly used.

A power supply converting AC line voltage to DC power must perform the following functions at high efficiency and at low cost:

BRIEF OVERVIEW OF THE 232 STANDARD

Officially, the 232 standard’s title is Interface Between Data Terminal Equipment and Data Circuit‑ Terminating Equipment Employing Serial Binary Data Interchange. The current revision is TIA/EIA‑232‑F (previously referred to as RS‑232). In the past, the term RS‑232 has been used interchangeably with its official name; however, the IRS prefix no longer is used and this document follows the new terminology. The standard discusses electrical specifications, i.e., the physical layer, protocol requirements, and specifies a 25‑pin D‑type connector that attaches to the cable. Discussion of mechanical characteristics is beyond the scope of this application report. Protocol is discussed briefly only for signals used with the 9‑pin connector.

The 232 standard was introduced in 1962 as an effort to standardize the interface for serial communication between the DTE and DCE. Early adopters emphasized the interface between the PC (DTE) and the modem (DCE). Wide acceptance of the PC quickly ensured that the 232 standard would become the industry standard it is today ‑ a low‑cost, serial interface. The mouse, plotter, printer, scanner, digitizer, external modems, and test equipment are examples of peripherals that connect to the 232‑compliant serial port.

Throughout the years, the 232 standard has been revised a number of times. Figure 1 shows several transition‑time vs data‑rate changes made to the standard. The latest revision makes 232 specifications agree with international standards ITU‑T V.24, V.28, and ISO/IEC 2110.

TECHNOLOGY EVOLUTION

The minimum driver output voltage level specified for loaded conditions is +5 V (see Table 1). For 232 drivers that transmit data through a cable, traditionally, this voltage level has been accomplished using multiple supplies to the discrete 1C. Because +1 2 V has been available readily on most systems, this voltage was applied to the 232 device to produce the necessary bus‑voltage swings. The trend in recent years is to reduce power dissipation, especially in portable applications. Therefore, board designers have been reducing power‑supply voltages, or eliminating them altogether, wherever possible. The lowering of supply‑voltage levels is a trait common to the semiconductor industry. Consequently, the burden of generating the necessary 232‑compliant voltage swings is on the 232 driver IC. This can be accomplished with an internal or external charge pump or dc‑to‑dc converter.

TI devices incorporate the 232 driver and charge pump in a single solution that makes it easy to design a circuit board using only a single supply. The new MAX3243, MAX3238, and MAX3221 devices operate from 3 V to 5.5 V and require only four small external capacitors (0.1 pl) for the charge‑pumping action. These devices are built in the state‑of‑the‑art LinBiCMOS process, making them the only 232‑compliant devices in the broad TI product portfolio with a wide low‑voltage supply range.

Figure 1. Changes in 232‑Standard Transition Times vs Data Rates

TIA/EIA‑232‑F places four constraints that limit the unit interval, which is the inverse of data rate, and transition time, defined by the slew rate. Referring to Figure 1, the first constraint is set by the top line. The standard specifies that for any data rate less than or equal to 40 bps, the transition time cannot exceed 1 ms. The second constraint is illustrated as the 4%‑limit line drawn diagonally from 40 bps to 200 kbps. This specifically states that for any unit interval less than or equal to 25 ms (equivalent to ‑:f40 bps data rate) but greater than or equal to 50 its (equivalent‑to <20 kbps data rate), the transition time must not exceed 4% of the unit interval. The third constraint shown pertains to the bottom line, which places a maximum limit of 30 V/11s on the driver output slew rate. This maximum slew rate translates to a minimum transition time of 0.2 Its because 232 voltage swings should never fall below 6 V (0.2 ~ts = 6 V/30 V/~ts). Finally, the fourth limit is data rate, which is widely known in the industry to be only 20 kbps. Hence, the dashed line in Figure 1.

ANSI TIA/EIA‑232‑F ELECTRICAL SPECIFICATION

It is evident from the specifications in Table 1 that the 232 standard was created when power considerations were not scrutinized closely and data signaling rates, although adequate at that time, were extremely slow when compared to today’s standards. To be compatible with the original standard, receiver inputs and driver outputs must tolerate ±25‑V voltage swings. Continually increasing demand for speed has forced discrete‑IC manufacturers to violate the 232‑standard maximum 20‑kbps specification at the expense of ignoring the 4% limit shown in Figure 1. Today, 250‑kbps data‑signaling rates are common. Table 1 presents key 232‑compliant specifications and features. Figure 2 graphically shows driver and receiver compatibility specifications.

Table 1. Key 232‑Standard Electrical Specifications

232‑STANDARD HANDSHAKING SIGNALS FOR DB9S CONNECTIONS

As mentioned previously, the 25‑pin connector is recommended in the 232 standard. However, in today’s applications, the 9‑pin D‑type connector, commonly known as the DB9S connector, is chosen by most users because not all handshaking signals are necessary in most applications. Typically, the handshaking signals used with the DB9S connector are sufficient to support error‑free data transmission. A description of the protocol signals and their corresponding D139S connector pins are presented in Table 2.

Table 2     232 Signal Description

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