Lecture 15: Point-to-Point Data Links

RS232 Interface

This interface, which dates back to the 1960s, was originally based on a 25 pin connector^[1]. In the following, signal names refer to the computer (or DTE) end of the link: for example, pin 2 is "Receive data" on a modem -- that is, an RS232-compliant cable is wired "straight-through". The major signals defined in RS232 are:

As mentioned, the RS232 interface was designed to connect a modem (DCE) to a computer (DTE). To make a direct (DTE-DTE) link, a null modem cable (or null modem adapter) must normally be used. This is sometimes called a crossover cable, since pin 2 at one end is wired to pin 3 at the other, etc.

^[1] 9 pin connectors are also used, but there is (perhaps surprisingly) no standard pin assignment for this -- the PC/AT pinout is obviously the most common.

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