The DTE/DCE Physical Interface

  • William J. Barksdale
Part of the Applications of Communications Theory book series (ACTH)


The interface between the DTE (data terminal equipment, e.g., terminal or computer) and the DCE (data circuit terminating equipment, e.g., modem) is of fundamental importance in data telecommunications since it not only transfers data, but also provides electrical grounding and a host of control and status indications in both directions. On the DTE side of the interface the input/output (I/O) port must be able to take data from the high-speed internal parallel bus of the user’s terminal and convert it to serial form at much lower speed in the proper format for transmission by the modem. It must be able to indicate when data is ready to be sent and then to send the data at the proper time and synchronization. Conversely, the I/O port must be able to take data from the modem at the proper time and synchronization, indicate when it is ready and when there are errors, and then convert this data from serial to parallel form for the internal data bus. The heart of most I/O hardware is an LSI chip called a USART or, if only asynchronous capability is provided, a UART. These devices provide the necessary serial/parallel conversion, clocking, synchronization, error detection, and modem control signals for an efficient interface. They are typically programmable, or at least strappable, and are controlled by the I/O driver routines of the DTE operating system or monitor. These routines normally either poll the USART to determine its status periodically, or else respond to software or hardware interrupts that the USART may produce.


Secondary Channel Master Station Physical Interface Line Driver Ring Indicator 
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  1. 1.
    Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange (EIA Standard RS-232C), EIA, Washington, D.C. (1969).Google Scholar
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    List of Definitions for Interchange Circuits between Data-Terminal Equipment and Data Circuit-Terminating Equipment (CCITT Recommendation V.24), ITU/CCITT, Geneva (1981).Google Scholar
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Suggested Readings

  1. M. Sargent III and R. L. Shoemaker, Interfacing Microprocessors to the Real World, Addison-Wesley, Reading, Massachusetts (1981).Google Scholar
  2. A nice, application-oriented paperback that discusses current loop and RS-232C modem interfacing with the 8251 USART.Google Scholar
  3. M. D. Seyer, RS-232 Made Easy,Prentice-Hall, Englewood Cliffs, New Jersey (1984). This entire book on RS-232 gives an extremely clear and simple explanation of how the interface operates, including various haywire configurations plus an extensive appendix on personal computer connections.Google Scholar

Copyright information

© Plenum Press, New York 1986

Authors and Affiliations

  • William J. Barksdale
    • 1
  1. 1.South TEC AssociatesHuntsvilleUSA

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