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14     Introduction 

Terminal to Modem (DCE) or Host 

Refer to the diagram below to understand why a null modem 
adapter may be needed when connecting the second serial port 
(SES2-AUX) to a modem or a DCE host.  The first serial port will 
connect directly without a null modem since it is a DTE device. 

 

 

Terminal to Host (DTE) or Printer 

Refer to the diagram below to understand why a null modem 
adapter may be needed when connecting the first serial port 
(SES1-EIA) to a host port without a modem (most serial printers 
are DTE devices and most hosts are not).  The second port (AUX) 
will connect directly, without a null modem, since it is a DCE 
device.