serial is getting pretty hard to use. lots of low-end motherboards otherwise perfect for plan 9, either don't have serial ports or have the irq mappings screwed up in the mp table. i'm batting 1.000 at home with terminal/ cpu server mb with screwed-up serial. all the machines i want to have serial consoles have two serial ports that work great. :-\
also, many of the cards supported by plan 9 are very expensive if you don't need 32 serial ports in a system and they don't support pcie. after a crash that scrolled right off the cga, i finally broke down and took a chance on the StarTech 4 port pci-e 16c950 card. (p/n pex4s950) from newegg http://www.newegg.com/Product/Product.aspx?Item=N82E16815158087&Tpk=startech%2bpex4s950 complete with octopus cable for $100. other than the fact that the cable and card need to be mashed together with excessive force, it works great. (i did need to remove the card from the case to apply enough pressure to make them.) the crystal speed is 1843200hz. patch uartpex4s950 is on sources. - erik