What is RS-232?

The last few articles ("Serial Port Technology") discussed serial connections from the point of view of the serial port's UART. Now we move outwards from that point to look specifically at RS-232, the most common mode of serial communication among personal computers.

After the serial port's UART has done its work by composing data for serial transmission, the data must be moved across the wires of a serial port. One way of doing so is called "RS-232." RS-232 is a species of serial connection described in a specification written by the Electronic Industries Association (EIA) which, in conjunction with the Telecommunications Industry Association, defines the standards for traditional serial data transfer. Formally, the RS-232 standard is called EIA/TIA–232–F, reflecting the initials of the organizations that administer it.

The RS-232 serial port specification describes the types of RS-232 communications equipment as well as the signalling, electrical, and mechanical characteristics of RS-232 serial ports.

Serial Communications Equipment

The RS-232 specification defines two types of communication equipment: Data Terminal Equipment, or DTE; and Data Circuit-Terminating Equipment, or DCE. These two types of equipment differ in pin-out assignments—for practical purposes a PC can be considered a DTE, and a modem a DCE. Usually an RS-232 link connects a DTE and a DCE. A link can also use a crossover cable (sometimes called a “null-modem cable”) to make the connection between two DTEs appear as if it were between a DTE and a DCE (as when connecting two PCs with a serial cable). DTE and DCE configurations for cables and connectors are shown below.

The next article will explain the meanings of the various signalling lines — TD, RD, DCD, etc.

All About Serial

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