Peter:
...
> The Pavilion a6345f now has a PCI card with two serial ports. If the
> SCSI adapter is not in the machine, kernel parameter
> --console=ttyS0,115200 provided the boot sequence via the serial
> crossover cable.
>
> If the SCSI card is added, nothing comes from the kernel. Therefore
> read
> https://www.gnu.org/software/grub/manual/grub/html_node/Serial-terminal.html
> and tried to configure output from GRUB.
>
> grub> serial
> error: serial port 'com0' isn't found.
>
> The kernel recognizes the serial ports and GRUB doesn't? A
> fundamental conflict between the SCSI card and other hardware?
> Suggestions for a next step?

Is it so that you get a working console on serial port if you don't
have the scsi card in the computer, but not if you add the scsi card ?
If so, try different pci slots for your scsi and serial cards,
it can possible be some interrupt or other conflict.

///
Since I don't know much about your kernel etc., and
https://www.gnu.org/software/grub/manual/grub/html_node/serial.html
doesn't reposnd.

A dual serial pci card is probably served by the parport_serial module.
It might be so that their serial ports appear as something else as
ttyS0 and ttyS1. Here an example:
# setserial -bg /dev/ttyS? 2>&1 | grep -v "No such"
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
/dev/ttyS4 at 0xc480 (irq = 17) is a 16650V2
/dev/ttyS5 at 0xc800 (irq = 18) is a 16650V2
/dev/ttyS6 at 0xc880 (irq = 19) is a 16650V2
/dev/ttyS7 at 0xcc00 (irq = 16) is a 16650V2
/dev/ttyS8 at 0xe480 (irq = 16) is a 16950/954
/dev/ttyS9 at 0xe488 (irq = 16) is a 16950/954
#
where ttyS0 is the onboard one, and pci-e cards ports
appears after the classic ones, se below:

From the setserial man page:
The "standard MS-DOS" port associations are given below:
/dev/ttys0 (COM1), port 0x3f8, irq 4
/dev/ttys1 (COM2), port 0x2f8, irq 3
/dev/ttys2 (COM3), port 0x3e8, irq 4
/dev/ttys3 (COM4), port 0x2e8, irq 3

Since grub is running before the kernel it doesn't know how the kernel
will assign the device names, so it might be so that
serial --unit=0
only is for a serial port at io-port 0x03f8 aka the classic DOS COM1
and the kernel uses ttyS0 for a port at another io-port or some
other confusion.

Regards,
/Karl Hammar