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Communications 11
DTE
SES1 EIA
Modem
Transmit
Receive
DTR
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin2
pin3
pin20
pin6
D
C
E
Host
DCE
SES2 AUX
Modem
D
C
E
Host
Receive
Transmit
DTR
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin2
pin3
pin20
pin6
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
SES2 AUX
Receive
Transmit
DTR
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin2
pin3
pin20
pin6
D
T
E
Host
DTE
SES1 EIA
D
T
E
Host
Transmit
Receive
DTR
DSR
Transmit
Receive
DTR
DSR
pin2
pin3
pin20
pin6
pin2
pin3
pin20
pin6
Serial
Printer
Serial
Printer