The new JetPort 5201 single-port RS232 serial device server is claimed to break the industry record of serial-to-Ether net performance by achieving a maximum of 460.8kbps serial speed in full duplex.

Intel Desktop Board D945GTPLKR Mainboard micro ATX i945G LGA775 Socket UDMA100, Serial ATA300 (RAID) Gigabit Ethernet FireWire video HD Audio (6channel) BOXD945GTPLKR.. . Price: $132

Korenix Technology expands the new JetPort 5601f RS232422485 3-in-one to fibre Ethernet device server to JetPort product line.

Magnum DX 800 and DX 40 Integrate IPEthernet Technologies With Existing Industrial Devices GarrettCom, Inc., is introducing a groundbreaking line of Serial-IP converters that extend security and resiliency and network management to distributed serial devices in power substations and other harsh industrial environments. The Magnum DX line o

Industrial Serial to Ethernet device servers are ideal for connecting serial-based software applications over Ethernet to your remote industrial serial devices.

New industrial interfaces, now available in the UK from Metrix Electronics, offer cost-effective conversion of Fieldbus or Ethernet data to standard serial standards

Then you should use a a serialEthernet device that includes such a driver. Writing such a driver for Linux is pretty a pretty nasty job (been there, done that). My ex-employer makes a whole line of such devices (as do other companies): link