AN2020| Application Note
Maxim/Dallas > App Notes > INTERFACE CIRCUITS Keywords: RS-232, autoshutdown, ESD protection, megabaud, serial port, rs232, rs 232
May 11, 2003
APPLICATION NOTE 2020
Choosing the Right RS-232 Transceiver
This article is intended to help the design engineer sort through the various features available in RS-232 interface products. The main features described are the regulated charge pump, Autoshutdown, RS-232 compatible versus compliant operation, ESD protection, and data rates including Megabaud operation.
RS-232 Background
Despite the development of newer digital interface standards, the humble RS-232 serial port is still a very popular means of data transfer. Robust and easy to use, the RS-232 interface is still an attractive alternative to the more demanding and temperamental digital interfaces. This application note describes key features and helps the designer choose the right RS-232 transceiver. Factors to consider include:
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Power supply requirements Power consumption/battery-life features (such as autoshutdown) Speed (data rates) Compliance with signal level standards Enhanced ESD protection Isolation features Package size
The Single-Supply RS-232 Transceiver
The RS-232 standard requires signal levels of +5V (minimum) for a low and -5V (minimum) for a high. A higher voltage swing ensures greater noise immunity after the signals travel through lossy cables to RS-232 receivers. Early RS-232 transceivers (such as the MAX1488 and MAX3185, which are still in use today) required dual power supplies. Most circuit boards included a negative power supply solely to power these devices. In 1987, Maxim introduced the MAX232, the first single-supply RS-232 device. The MAX232 (and other single supply devices) includes two charge pumps. The first charge pump doubles +5V to approximately +10V and the second charge pump inverts the +10V charge-pump output to -10V. Since these charge pumps are unregulated, the positive and negative output voltages of the transmitters often drop well below +10V and -10V. The depth of the power supplies drop is a function of both the capacitance of the cable driven by the transmitters and the data rate. The MAX232 quickly became an industry standard
. Many board designers still use the MAX232 today, despite the fact that single-supply devices have undergone extensive improvements over the years. Understanding the features of new devices will help the design engineer choose the best parts for the application.
RS-232 Transceivers Powered by Low-Voltage Single Supplies
As power-supply voltages dropped from +5V to +3.3V and lower, single-supply RS-232 transceivers kept pace. The transceivers that are powered from voltages ranging from 3.0V to 5.5V use two regulated charge pumps instead of the unregulated types used in the parts powered by only +5V. Rather than double the input voltage, a single regulated charge pump boosts the input voltage to approximately +5.4V. A second regulated charge pump
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