The RS-232C (Recommended Standard 232, version C) is a long-established standard for serial communication between two devices. For many years, it was the most common interface for connecting computers to peripherals like modems, printers, and laboratory instruments. It typically uses a D-subminiature connector, most often a 9-pin (DE-9) or 25-pin (DB-25) version. RS-232C is a point-to-point, asynchronous protocol. "Point-to-point" means it is designed for communication between only two devices. "Asynchronous" means there is no shared clock signal; instead, the timing is handled by using a pre-agreed baud rate (communication speed) and framing the data with a start bit and one or more stop bits.
RS-232C has several limitations that have led to its replacement by USB and Ethernet in most modern applications. Its communication lines are "single-ended," meaning voltages are referenced to a common ground. This makes it susceptible to noise and limits the communication distance to around 15 meters (50 feet) at typical speeds. Furthermore, its speeds are relatively low compared to modern standards. Despite these limitations, RS-232C is simple, reliable for short distances, and is still found on many industrial and scientific instruments. To establish a connection, both devices must be configured with identical communication parameters: baud rate, data bits (usually 8), parity (e.g., None, Even, or Odd), and stop bits (usually 1).
It was commonly used for computer interfaces before the prevalence of LAN and USB. Communication speeds include 2400 and 9600 bps. Because the communication lines are single-ended rather than differential signals, the communication distance is short and vulnerable to noise. Furthermore, it is limited to one link per port because the number of connections is 1:1. Communication depends on the alignment of communication speed (bits per second, bps); data bits; parity; stop bits; flow control; and other device settings.