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Directional Signals

The Directional Nature Of The RS-232 Signals

Here's an interesting thought for your consideration. The direction of the control signals for the pins on the DTE side of the connection is the opposite to the direction of the same numbered pin on the DCE side of the conversation. Think about the DTR line. This line is connected to Pin 20 on the connector for both DCE and DTE. The control signal, however, is asserted by the DTE and received by the DCE ( the signal direction is ‘To DCE’). Therefore, it is only with respect to the DCE that we could say, "If a signal is received on Pin 20 it indicates that the line is asserted". If a signal were to be received by Pin 20 on the DTE then the connection is misconfigured. Pin 20 on the DTE is a signal sender. It can’t do anything if a signal is sent "to" it. What do you know about Pin 20 on the DTE? You know that if the DTE is operational it asserts voltage on the line (it sends the control signal). The DTE is, in effect, the sending side for the DTR signal. This sense of "opposite direction" implies that one side sends the control signal and the other side receives and responds to the control signal. Now, with respect to a DCE-to-DTE connection this whole point is essentially moot. The cable is wired straight through; Pin 1 is connected to Pin 1, 2 to 2, and so on, with Pin 25 connected to Pin 25. The differentiation between the sending and receiving function for a pin is the differentiation between a DTE and a DCE.