RS-232 signaling

RS-232, which stands for "Recommended Standard 232," is a standard for serial communication that was first introduced by the Electronic Industries Association (EIA) in 1960. RS-232 has been widely used in the past for connecting various types of equipment, such as computers, modems, printers, and other peripherals, but has largely been replaced by more modern communication standards like USB, Ethernet, and wireless technologies.

The following presents an overview of RS-232 signaling.

Electrical Characteristics

RS-232 communication uses voltage signaling. It defines two voltage levels: a "mark" or "space." A "mark" is typically represented by a voltage in the range of -3 to -15 volts, while a "space" is in the range of +3 to +15 volts. These voltage levels are often referred to as "binary 1" and "binary 0," respectively

Serial Communication

RS-232 is a serial communication standard, meaning data is transmitted one bit at a time. It uses a start bit, data bits (usually 8 bits), an optional parity bit for error checking, and one or more stop bits.

Data Transmission

Data is transmitted in the "least significant bit first" order. Data can be sent asynchronously (without a clock signal) or synchronously (with a clock signal). Asynchronous communication is more common.

Baud Rate

Baud rate refers to the number of signal changes (or symbol changes) per second. It is often expressed in bits per second (bps).

Common baud rates include 300, 1200, 2400, 9600, 19200, and 115200 bps, with a baud rate of 9600 being the most commonly used in the industry.

Handshaking

RS-232 supports various handshaking protocols to control data flow between devices. Common handshaking methods include RTS/CTS (Ready to Send/Clear to Send) and DTR/DSR (Data Terminal Ready/Data Set Ready).

Connectors and Cabling

RS-232 uses a variety of connectors, with the most common being the DB-9 and DB-25 connectors. The cabling consists of at least three wires: transmit (TX), receive (RX), and ground (GND). Many RS-232 cables use a null modem configuration, which swaps the transmit and receive lines between devices.

Applications

RS-232 has historically been used for connecting devices like modems, printers, and terminals to computers. It is still used in some specialized applications, such as industrial control systems and legacy equipment.

RS-232 is still used in certain industries where reliability and simplicity are more critical than high-speed data transfer. For example, it's commonly found in industrial control and automation systems.

Limitations

RS-232 has several limitations, including limited cable length (typically up to 50 feet), susceptibility to electrical interference, and relatively slow data transfer rates compared to modern standards.

Replacement by USB

USB (Universal Serial Bus) has largely replaced RS-232 in most consumer and computer applications due to its higher speed, ease of use, and plug-and-play capabilities.

RS-232 decoding with PicoScope

RS-232 serial decoding is included in PicoScope as standard. The decoded data can be displayed in the format of your choice: In Graph, In Table, or both at once.

In Graph format shows decoded data in Binary, Hex, Decimal, or ASCII format, aligned with the analog waveform, on a common time axis. Decoded data can be zoomed and correlated with acquired analog channels to investigate timing errors or other signal integrity issues that are root cause of data errors.

In Table format shows a list of the decoded packets, showing data values with the packet start and stop times.

The PicoScope RS-232 decoder can also handle similar serial data standards such as RS-422, RS-485 and UART.