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DB-25 Connection

The Physical Cable Connector

The original "standard" connector for RS-232 is the DB-25. A DB-25 is a trapezoidal shaped, 25 pin connector. The DTE (the computer) uses the male connector (with the pins sticking out) and the DCE (the modem) uses the female connector. (The previous topic discusses the differences between DTE AND DCE). The trapezoidal shape of the connector keeps the user from plugging it in upside down.

You'll notice that you are viewing the connector with the shorter side of the trapezoid at the bottom. In this orientation, Pin 1 is in the upper right. The pins count up across the top row and then start over on the bottom right, continuing the count across the bottom row to the left. Pin 25 is on the lower left.

A brief note on the convention used in the diagrams in this book: the fact that this is a female connector is represented by the pins being shown as white dots. If this were a male DB-25 the diagram would show the pins as black dots.

If you think about the pinout of the DCE side of the connection (as shown above) then you'll realize that the DB-25 male connector has a mirror image of the pinout of the female connector. In order for the two connectors to plug together it's necessary for the male connector to be exactly opposite in its pinout from the female connector. Pin 1 on the male connector is in the upper left (when viewed horizontally). In this drawing you are looking at the DTE DB-25 port on the back of a computer (the DB-25 on the right). You are holding the DCE cable so that you are looking at the end of the connector. The two faces that are shown in the picture will plug into each other. Compare the two pinouts. You see that when you plug the female end (on the left) into the male connector (on the right) you'll have to mate the trapezoid shape connector -- Pin 1 will mate with Pin 1, and so on.

As an engineer responsible for working with RS-232 connections it is critical that you can immediately identify the pinouts on a connector. My wife helped me know where Pin 1 was located. She told me, "Men always think they're right, but they're not." Now, if men think they are always right, but they're not, then they must be left, and women must be right. Right? The male connector has Pin 1 on the left, the female connector has Pin 1 on the right. Hopefully, now, you'll never forget.

A description of the pinouts on the 25 line RS-232 cable will help explain why th