The serial directive tells Heartbeat to use the specified serial port(s) for its communication. The parameters to the serial directive are the names of tty devices suitable for opening without waiting for carrier first. On Linux, those ports are typically named /dev/ttySX.
A few sample serial directives are shown below:
serial /dev/ttyS0 /dev/ttyS1 # Linux serial /dev/cuaa0 # FreeBSD serial /dev/cua/a # Solaris
The baud directive is used to configure the baud rate for the port(s) if the baud directive is specified before the serial directive, otherwise the default baud rate will be used.