The ASYNC interface consists of separate read and transmit lines. There are two separate modem control signals whose output values can be controlled by the physical device driver:

• Data Terminal Ready (DTR)
  
• Request To Send (RTS)

There are four separate modem control signals whose input values are available to the physical device driver:

• Data Set Ready (DSR) 
  Clear To Send (CTS)
  
  Data Carrier Detect (DCD), also known as Receive Line Signal Detect (RLSD)
  
  Ring Indicator (RI)

  The receive and transmit data lines have the following hardware characteristics:

  • Logical 1 (Marking). More negative than -3 Volts. This state could mean no data. 
    Logical 0 (Spacing). More positive than +3 Volts. This state could mean break condition.

    The modem control signal lines have the following hardware characteristics:

    • Function ON, when more positive than +3 Volts. 
      Function OFF, when more negative than -3 Volts.

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