Digi EtherLite is great for putting serial ports out on a network cable (unless you have old IBM terminals). This is currently our favorite solution (on a cost / convenience basis) for 8 ports and up. Equinox makes a similar device at a slightly lower price but I haven't tried it yet.

For 4 ports we have been using a Stallion 4-port Easy-IO board and it's reasonably priced ($295 list - less on the street). Cyclades makes an almost identical board using the same chip set which is more popular among Linux users, but it isn't carried by our usual distributors.

Siig makes a 2-port PCI sold at Frys for around $75, but it doesn't support Linux at all. Frys carries another brand which claims it works with Linux, but it didn't when I tried it.

They're very flexible and can handle complex situations. The truth is, though, they're overkill for the kind of stuff our clients do. The Etherlites are more appropriate since they cost less and the ports behave more like regular serial ports.

Our typical multiport client is a medical testing lab that runs either Hex or CIS software (and no other applications at all). These are LIS (Laboratory Information Systems) extended with results forms printing and electronic billing. The host/servers are all SCO Unix, though Hex is starting to deploy Linux. Typical usage of serial ports would be:

• 4 or 5 medical testing machines that download results to the LIS.
  
• 4 or 5 Wyse 60 terminals
  
• A barcode printer
  
• 4 or 5 dot matrix printers.
  
• 1 customer call-in modem.
  
• 3 call-out modems (fax, remote printing, billing)

Configuration is rarely changed and user interaction is quite simplistic. The two principle requirements are: no downtime; no lost data, ever.