Designing Embedded Hardware

Old Faithful—RS-232C

RS-232C is a serial communication interface standard that has been in use, in one form or another, since the 1960s. RS-232C is used for interfacing serial devices over cable lengths of up to 25 meters and at data rates up to 38.4kbps. You can use it to connect to other computers, modems, and even old terminals (useful tools for monitoring status messages during debugging). In days of old, printers, plotters, and a host of other devices came with RS-232C interfaces. With the need to transfer large amounts of data rapidly, RS-232C is being supplanted as a connection standard by high-speed networks, such as Ethernet. However, it can still be a useful and (importantly) simple connection tool for your embedded system.

RS-232C is unbalanced, meaning that the voltage level of a data bit being transmitted is referenced to local ground. A logic high for RS-232C is a signal voltage in the range -5 to -15V (typically -12V), and a logic low is between +5 and +15V (typically +12V). So, just to make that clear, an RS-232C high is a negative voltage, and a low is a positive voltage, unlike the rest of your computer’s logic.

The terminology used in RS-232C also goes back to the 1960s. In those days of mainframes, a high (1) was called a “space” and a low (0) was called a “mark.” You’ll still find these terms kicking around in RS-232C, where you’ll hear phrases like “mark parity” and “space parity.” It’s also not unheard of to see RS-232C systems still using 7-bit data frames ...