If you have to do this, the best option is to replace the card with the current 3Com card (ETC part number M841) and also get the packet driver update kit (part number 4152S1014). These are available from an ETC Dealer or Service Center.
We used the 3Com 3c900-CMB card with the 3C90XPD.COM driver version 4.0d located in the C:\ETC directory on the Obsession II remote processor unit. We used to use the 3Com 3c900-COMBO card with the 3C90XPD.COM driver version 2.0c.
If you use a different ethernet card, ETC can't guarantee that it will work with the Obsession II system, and cannot provide support on communication problems. At the very least, the card must provide a DOS driver which you must copy into the c:\etc directory and name 3C90XPD.com for it to be loaded properly by the AUTOEXEC.BAT file. This will overwrite the current 3Com driver, so you must first rename the 3Com driver file to some temporary name (eg. 3COMDRIV.COM). This is probably a good idea in case the card you install doesn't work and you have to go back to a 3Com card.