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Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 3024
Keywords: RS-232, RS232, IrDA, UART, serial, interface, endec
APPLICATION NOTE 3024
IrDA and RS-232: A Match Made in Silicon
By: Ted Salazar, Applications Engineering Manager
Jul 22, 2004
Abstract: IrDA is a standardized, wireless, infrared data communications method. Modern products often
support RS-232 alongside IrDA, meeting legacy requirements. This application note describes ways to
achieve both interfaces with a single UART.
Background
The term data-communications interface is ambiguous, but it can be resolved into three easily
understood layers: the physical layer defines electrical characteristics and hardware communication
speeds, the protocol layer defines the data format, and the application layer employs this data format to
perform tasks in a given device. The application layer IrTran-P in IrDA, for example, is used in digital
cameras and other imaging devices to transfer pictures.
RS-232 encompasses only the physical layer, but IrDA encompasses all three (physical, protocol, and
application). IrDA includes several different physical layers: SIR (115kbps), MIR (1Mbps), FIR (4Mbps),
and VFIR (16Mbps). Because RS-232 and IrDA SIR use the same data framer (commonly known as a
UART), this article considers only RS-232 and the IrDA SIR physical layer.
Because RS-232 has (until recently) been included in almost every computer for the last three and half
decades, it is the most widely used physical layer in asynchronous serial communications. Most personal
computers are replacing RS-232 with the Universal Serial Bus (USB), but RS-232 remains common in
network equipment, industrial-control equipment, and point-of-sale equipment (Table 1).
Table 1. Specifications important to the TIA/EIA-232-F physical layer
Specification Minimum Maximum
Number of transmitters per data line
1 TX
Number of
receivers per data line 1 RX
Cable length
C <
2500pF
Data rate
20kbits/sec*
Loaded Driver Output
Range (0V Offset) ±5V ±15V
Maximum Driver Short Circuit Current
100mA
TX Load Resistance 3kΩ
7kΩ
Maximum Slew Rate
<30V/us
RX Input Range -
25V +25V
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