TITLE
    RS-232: DTEs, DCEs and Pin Signal Sequences
Article ID:
Created:
Modified: 		1376
10/23/85
3/31/00
TOPIC

    This article discusses RS-232 DTEs, DCEs and Pin Signal Sequences.

DISCUSSION

    Note: Throughout this discussion acronyms will be used in place of full names. Since all knowledgeable data communications personnel use these acronyms, you should practice using them.

    DTE and DCE
    According to the RS-232 standard, all devices involved in data communications can be classified into two groups:

    DTE - Data Terminal Equipment: those devices that originate the data or are its final destination. Examples would be computers, printers, tape drives, etc.

    DCE - Data Communications Equipment: those devices whose function it is to communicate the data between the DTEs. Examples would be modems, data multiplexers, and digital service units.

    The RS-232 cabling between a DTE and a DCE is straight through. That is, pin 1 from the DTE plug goes to pin 1 on the DCE socket, pin 2 goes to pin 2, and so on. If a pin is an output for a DTE, then it is an input for a DCE (and vice versa).

    RS-232 PIN GROUPS (three main groups)

    1. CONTROL pins carry signals that indicate and control the state of the DTE/DCE interface.
    2. DATA pins carry data signals.
    3. CLOCK pins carry the clock signals necessary for synchronous data communications.

    Because Half Duplex has to control line direction, it requires more RS-232 pins. The following is the sequence of RS-232 signals which are usually used for Half Duplex .

    HALF DUPLEX SIGNAL SEQUENCE

    Sequence   Pin   Full Signal Name             Output From  Input To

    (1)      20    Data Terminal Ready (DTR)    DTEs         DCEs
    (2)      6     Data Set Ready (DSR)         DCEs         DTEs
    (3)      4     Request to Send (RTS)        Tx DTE       Tx DCE
    (4)      8     Rx'd Line Sig Detect (DCD)   Rx DCE       Rx DTE
    (5)      5     Clear to Send (CTS)          Tx DCE       Tx DTE
    (6)      2     Tx Data (TD)                 Tx DTE       Tx DCE
    (7)      3     Receive Data (RD)            Rx DCE       Rx DTE
    Constant   7     Signal Ground (SG)            n/a           n/a

    Fortunately, most Apple products use Full Duplex and so require fewer pins. The following is the sequence of RS-232 signals which are often used for Full Duplex.

    FULL DUPLEX SIGNAL SEQUENCE

    Sequence   Pin   Full Signal Name       Output From  Input To

    (1)      20    Data Terminal Ready    DTEs         DCEs
    (2)      6     Data Set Ready         DCEs         DTEs
    (3)      2     Tx Data                Tx DTE       Tx DCE
    (3)      3     Receive Data           Rx DCE       Rx DTE
    Constant   7     Signal Ground           n/a           n/a

Document Information
Product Area: Computers; Peripherals
Category: Cables
Sub Category: Serial
Keywords: HTS, CNFG

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