
Communications 11
DTE
Modem
Transmit
Receive
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin20
D
C
E
Host
DCE
Modem
D
C
E
Host
Receive
Transmit
DTR
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin20
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.
DCE
Receive
Transmit
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin20
D
T
E
Host
DTE
D
T
E
Transmit
Receive
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin20
Serial
Printer
Serial
Printer
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