serial console on a machine without a serial port, but with a USB port

[John .]

I need to have access to the console on my server, so that I can do
stuff like grab the output of a crash or, if it halts whilst booting
up, to take remedial action. I had planned on doing this from a much
older machine whose sole function is to provide that capability. This
older machine will be remotely accessible, and it sits next to the
server machine, on its own real IP address, running ssh and maybe tip
or whatever to read the console of the other machine. Sort of like a
poor man's ILO.

The problem is, the server is a much newer machine and it has no
serial port - just USB. Is the only way round this to install a serial
card? Would a serial-to-usb converter work? Will usb to usb work? Both
machines have USB.

on the server machine with a generic kernel (7.2-STABLE), I have this:

 dmesg | grep -E "^sio[0-9]"
sio0: configured irq 4 not in bitmap of probed irqs 0
sio0: port may not be enabled
sio0: configured irq 4 not in bitmap of probed irqs 0
sio0: port may not be enabled
sio0: <16550A-compatible COM port> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
sio0: type 16550A
sio0: [FILTER]
sio1: configured irq 3 not in bitmap of probed irqs 0
sio1: port may not be enabled
sio0: configured irq 4 not in bitmap of probed irqs 0
sio0: port may not be enabled
sio0: configured irq 4 not in bitmap of probed irqs 0
sio0: port may not be enabled
sio0: <16550A-compatible COM port> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
sio0: type 16550A
sio0: [FILTER]
sio1: configured irq 3 not in bitmap of probed irqs 0
sio1: port may not be enabled

and /dev has no sio* entries

cheers
-- 
John