CONTENTS

 1     Scope
        1.1     Range of application
        1.2     Applicable electrical characteristics
        1.3     Applicable mechanical characteristics
        1.4     Allocation of functionalities
 2     Line of demarcation
 3     Definitions of interchange circuits
        3.1     Circuit 102 – Signal ground or common return
        3.2     Circuit 102a – DTE common return
        3.3     Circuit 102b – DCE common return
        3.4     Circuit 102c – Common return
        3.5     Circuit 103 – Transmitted data
        3.6     Circuit 104 – Received data
        3.7     Circuit 105 – Request to send
        3.8     Circuit 106 – Ready for sending
        3.9     Circuit 107 – Data set ready
       3.10     Circuit 108/1 – Connect data set to line
       3.11     Circuit 108/2 – Data terminal ready
       3.12     Circuit 109 – Data channel received line signal detector
       3.13     Circuit 111 – Data signalling rate selector (DTE source)
       3.14     Circuit 112 – Data signalling rate selector (DCE source)
       3.15     Circuit 113 – Transmitter signal element timing (DTE source)
       3.16     Circuit 114 – Transmitter signal element timing (DCE source)
       3.17     Circuit 115 – Receiver signal element timing (DCE source)
       3.18     Circuit 116/1 – Back-up switching in direct mode
       3.19     Circuit 116/2 – Back-up switching in authorized mode
       3.20     Circuit 117 – Standby indicator
       3.21     Circuit 118 – Transmitted backward channel data
       3.22     Circuit 119 – Received backward channel data
       3.23     Circuit 120 – Transmit backward channel line signal
       3.24     Circuit 121 – Backward channel ready
       3.25     Circuit 122 – Backward channel received line signal detector
       3.26     Circuit 125 – Calling indicator
       3.27     Circuit 126 – Select transmit frequency
       3.28     Circuit 128 – Receiver signal element timing (DTE source)
       3.29     Circuit 131 – Received character timing (DCE source)
       3.30     Circuit 133 – Ready for receiving
       3.31     Circuit 134 – Received data present
       3.32     Circuit 135 – Received energy present
       3.33     Circuit 137 - Transmitted character timing (DTE source)
       3.34     Circuit 138 - Transmitted character timing (DCE source)
       3.35     Circuit 140 – Loopback/Maintenance test
       3.36     Circuit 141 – Local loopback
       3.37     Circuit 142 – Test indicator
 4     Operational requirements
        4.1     Data circuits and timing circuits
                  4.1.1     Data circuits
                  4.1.2     Timing circuits
        4.2     Control and indication circuits
                  4.2.1     Operation of circuits 107, 108/1 and 108/2
                  4.2.2     Circuit 125 - Calling indicator
                  4.2.3     Usage of circuit 126
                  4.2.4     Circuit 140 - Loopback/Maintenance Test
        4.3     Miscellaneous
                  4.3.1     Interrelationship of circuits 103, 105 and 106
                  4.3.2     Idle periods
                  4.3.3     Clamping
        4.4     Circuit failures (electrical)
        4.5     Provision of interchange circuits in DCEs and DTEs
        4.6     Data flow control
        4.7     Power-up considerations
 5     References