RS-232 Electronic Design & Firmware
by Embedded Design Engineer John Heritage


RS-232 is one of the very first serial protocols I leaned fully and worked with back in the 1980’s. My BBC B Computer back in the day had a serial port and was technically an “RS-432” port, electrically different to, but compatible with, RS-232 serial ports found on today’s PCs. Many embedded micro-controllers today still use this UART port for serial communication, and I often design and program with the RS232 port.

TIA/EIA-232-F (typically referred to as RS-232) is a common interface that can be found on almost every personal computer. RS-232 is a complete standard, not only including electrical characteristics, but physical and mechanical characteristics as well, such as connection hardware, pin-outs, and signal names. A point-to-point interface, RS-232 is capable of moderate distances at speeds up to 20Kbps. While not specifically called out in the specification, speeds of greater than 115.2Kbps are possible, provided that connections are short and proper grounding is used. Cable lengths of 30 feet are common, and cables of over 200 feet can be attained with low-capacitance cable.

An RS-232 bus is an unbalanced bus capable of full-duplex communication between two receiver/transmitter pairs, named data terminal equipment (DTE) and data communication equipment (DCE). Each one has a transmit signal that is connected to the receive signal on the other end. As such, there is a pin difference between the two sides. (Your PC is a DTE, while the connected peripheral is DCE.)

Each transmitter sends data by varying the voltage on the line. A voltage higher than 3V is a binary zero, while a voltage less than –3V is a binary one. Between these voltages, the value is undefined. To convert from logic levels (0 and 5V) to these levels and back, an RS-232 conversion IC, such as the 1488, 1489, or ubiquitous MAX232, can be used.

Typical RS-232 communication consists of a start bit, data bits, parity bits (if any), and stop bit(s). When communicating with PCs, the typical format is eight data bits, no parity, and one stop bit (8N1). Seven data bits, even parity, and one stop bit (7E1) is also common. A start bit is often a zero and a stop bit is often a one, as shown in Figure 1. The official specification does not delineate any communications protocol, including the use of start/stop bits.

Many embedded systems that use the RS-232 bus either interface with PCs or PC peripherals such as modems. Other systems use RS-232 so that bus traffic can be monitored easily with an inexpensive protocol analyzer or a PC equipped with two serial ports.

Almost every microcontroller vendor has products that include hardware support for RS-232, called Universal Asynchronous Receiver Transmitters (UARTs). UARTs are often interrupt-driven and capable of speeds up to 115.2Kbps with little software overhead, although this varies by architecture.