@jomofcw dosbox.conf says that about the serial parameters :
[serial]
serial1: set type of device connected to com port.
Can be disabled, dummy, modem, nullmodem, directserial.
Additional parameters must be in the same line in the form of
parameter:value. Parameter for all types is irq (optional).
for directserial: realport (required), rxdelay (optional).
(realport:COM1 realport:ttyS0).
for modem: listenport (optional).
for nullmodem: server, rxdelay, txdelay, telnet, usedtr,
transparent, port, inhsocket (all optional).
Example: serial1=modem listenport:5000
Possible values: dummy, disabled, modem, nullmodem, directserial.
serial2: see serial1
Possible values: dummy, disabled, modem, nullmodem, directserial.
serial3: see serial1
Possible values: dummy, disabled, modem, nullmodem, directserial.
serial4: see serial1
Possible values: dummy, disabled, modem, nullmodem, directserial.