rs-232 man page on BSDOS

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RS-232(7)		     BSD Reference Manual		     RS-232(7)

NAME
     RS-232 - RS-232 signal names and connector configurations

DESCRIPTION
     The signal names:

     ---------------------------------------------------------------------------
     DB-25   DB-9			       Source
      Pin     Pin   Name    EIA	    CCITT    DTE   DCE		   Signal
     ---------------------------------------------------------------------------
       1       -      PG      AA     101      -	    -	      Frame Ground
       2       3      TD      BA     103      ->	      Transmit Data
       3       2      RD      BB     104	    <-	      Receive Data
       4       7      RTS     CA     105      ->	     Request To Send
       5       8      CTS     CB     106	    <-	      Clear To Send
       6       6      DSR     CC     107	    <-	     Data Set Ready
       7       5      SG      AB     102      -	    -	      Signal Ground
       8       1      DCD     DF     109	    <-	   Data Carrier Detect
       9       -      POS     -	      -		    <-	Pos. DC Test Voltage *
      10       -      NEG     -	      -		    <-	Neg. DC Test Voltage *
      11       -      -	      -	      -	      -	    -	       Unassigned
      12       -      SDCD    SCF    122	    <-	Sec. Data Carrier Detect
      13       -      SCTS    SCB    121	    <-	 Secondary Clear To Send
      14       -      STD     SBA    118      ->	 Secondary Transmit Data
      15       -      TC      DB     114	    <-	     Transmit Clock
      16       -      SRD     SBB    119	    <-	 Secondary Receive Data
      17       -      RC      DD     115	    <-	      Receive Clock
      18       -      LL      LL     141      ->    -	     Local Loopback
      19       -      SRTS    SCA    120      ->	  Sec. Request To Send
      20       4      DTR     CD     108.2    ->	   Data Terminal Ready
      21       -      SQ      CG     110	    <-	Signal Quality Detect **
      22       9      RI      CE     125	    <-	     Ring Indicator
      23       -       -      CH CI  111 112  ->    <-	   Data Rate Selector
      24       -      SCTE    DA     113      ->	 External Clock Transmit
      25       -      TM      TM     142	    <-	      Test Mode ***
     ---------------------------------------------------------------------------

     *	   - These signals are officially Reserved, but usually used for test
	   voltages.

     **	   - This signal may be used as a local loopback signal in RS-232-D,
	   if it is not being used as SQ.

     ***   - This signal was Unassigned in RS-232-C, but commonly used as BUSY
	   indicator.

EXPLANATION OF SIGNAL USAGE
     A brief explanation of common signal usage by UNIX systems, referenced by
     RS-232-D Signal Name, follows.  Note that all signals are discussed from
     the DTE point of view.

   PRIMARY SIGNALS

     PG	    Protective Ground (Often referred to as Frame Ground) - should be
	    connected to the equipment frame, and is where the shield should
	    be connected if shielded cable is being used to reduce RFI noise.
	    This signal should only be connected at the DTE end.

     SG	    Signal Ground - provides d.c. reference potential for all signals,
	    including data, timing, and control operations. Required for all
	    connections.

     TD	    Transmit Data - data signals originating by the DTE, including
	    signals to be transmitted via a data channel for receiving sta-
	    tions, data to be passed to the DCE for testing and maintenance
	    purposes, and signals for programming or control of DCE equipment.

     RD	    Receive Data - data signals from the DCE, including responses to
	    data channel signals, test signals, and programming or control
	    signals from the DTE equipment.

     RTS    Request To Send - Defined by RS-232-D as a control signal used to
	    prepare the DCE for transmission of data. Commonly used by UNIX
	    systems as Ready To Receive, i.e., the DTE is ready to receive da-
	    ta from the DCE when RTS is in the on state.

     CTS    Clear to Send - the DCE is ready to receive data or control trans-
	    missions from the DTE.

     DCD    Received Line Signal Detector - commonly called Data Carrier De-
	    tect. Signal from the DCE indicating its data channel signal is
	    within appropriate limits, e.g. when DCE is a modem, carrier sig-
	    nal is present and modem is ready to communicate with other modem.

     DTR    Data Terminal Ready - signal from DTE to place DCE in on-line or
	    off-line state.

     DSR    Data Set Ready - the DCE signals the DTE that it is powered up,
	    connected, and ready to receive signals from the DTE (i.e. is not
	    currently conducting a data transmission, test, or dial opera-
	    tion).  Often used in conjunction with the DTR signal.

     RI	    Ring Indicator - signal used by an auto-answer modem to inform the
	    DTE that a ring signal is present on the telephone line. Most mod-
	    ern modems indicate call-progress on the TD line, and RI is no
	    longer used by most RS-232 circuits.

   SYNCHRONOUS COMMUNICATION SIGNALS

     STD    Secondary Transmit Data - Equivalent to TD, except it is used to
	    transmit data over the backward channel in synchronous circuits.

     SRD    Secondary Receive Data - Equivalent to RD, except it is used to
	    receive data over the backward channel in synchronous circuits.

     SRTS   Secondary Request To Send - Equivalent to RTS, except it is used
	    to control the backward channel transmit function in synchronous
	    circuits.

     SCTS   Secondary Clear To Send - Equivalent to CTS, except it is used to
	    control whether the DCE is conditioned to transmit data on the
	    backward channel in synchronous circuits.

     SDCD   Secondary Received Line Signal Detector - Equivalent to DCD, ex-
	    cept it is used to indicate whether the backward channel is in ap-
	    propriate limits in synchronous circuits.

     TC	    Transmit Clock - timing signal from the DTE that should be ON and
	    OFF for approximately equal periods of time. The DTE should pro-

	    vide a data signal on TR while the TC signal is ON.

     RC	    Receive Clock - timing signal from the DCE that should be ON and
	    OFF for approximately equal periods of time. A transition from ON
	    to OFF should coincide with the center of each data segment of the
	    RD circuit.

     SCTE   Serial Clock Transmit External - provides DCE with signal timing
	    information. This signal should be ON and OFF for approximately
	    equal periods of time, and the transition from ON to OFF condition
	    should coincide with the center of each data segment of the TD
	    circuit.

     DRS    Data Rate Selector - signal used by both the DTE and DCE to select
	    one of the two data signalling rates of a dual rate synchronous
	    DCE.

   TEST SIGNALS

     POS    Positive DC Test Voltage - an unofficial use of pin 9 in a DB-25
	    RS-232 circuit, used to provide positive test voltage from the
	    DCE.

     NEG    Negative DC Test Voltage - an unofficial use of pin 10 in a DB-25
	    RS-232 circuit, used to provide negative test voltage from the
	    DCE.

     LL	    Local Loopback - new in RS-232-D, initiates a local loopback test
	    condition in the DCE.

     SQ	    Signal Quality Detect - indicates whether there is a probability
	    of error in the data received in the data channel. Used prior to
	    RS-232-D.  If this signal is not being used for SQ, RS-232-D de-
	    fines its use as Remote Loopback.

     TM	    Test Mode - new in RS-232-D. Signal indicates if a maintenance
	    condition exists in the DCE. Prior to RS-232-D, this signal was
	    undefined, but commonly used as a BUSY indicator by the DCE.

SEE ALSO
     EIA Standard EIA-232-D
     CCITT Fascicle VIII.1-V.24

AUTHOR
     Stephen Diercouff	(sgd@tfm.com)

4.4BSD				March 18, 1993				     3
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