Commodore 64 Handbook page 84

indicate whether the DCE is conditioned to transmit data on
the backward channel.
Circuit 122 - Supervisory Carrier Detect
Equivalent to circuit 109, except that it is used to
indicate whether the received backward channel line signal
is within appropriate limits.
Circuit 123 - Backward Channel Signal Quality Detector
Equivalent to circuit 110, except that it is used to
indicate the signal quality of the received backward channel
1ine signal .
Circuit 124 - Select Frequency Groups
Used to select the desired -frequency groups available on the
DCE.
Circuit 125 - Calling Indicator
Indicates whether a calling signal is being received by the
DCE.
Circuit 126 - Select Transmit Frequency
Used to select the required transmit -frequency of the DCE.
Circuit 127 - Select Receive Frequency
Used to select the required receive frequency of the DCE.
Circuit 128 - Receiver Signal Element Timing
Provides the DCE with signal element timing information.
Circuit 129 - Request to Receive
Used to control the receive function of the DCE.
Circuit 130 - Transmit Backward Tone
Controls the transmission of a backward channel tone.
Circuit 131 - Received Character Timing
Provides the DTE with character timing information.
Circuit 132 - Return to Non-Data Mode
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