Xyplex 1608 for PPP over IP

Hey guys

I am interested in taking five computers and sending data through their
com ports, give that data an ip address with a terminal server, and
then send the data to a cisco switch for switching and routing
purposes.

The switch is set up fine, now i need to worry about the computer's
connection to the xyplex. Here is the schematic:

PC1 }--
PC2 | -->
PC3 | --->
PC4 | ---> Xyplex 1608A ---> Cisco switch ---> Cisco router
PC5 }--

the five computers have been divided so that some belong to one network
and some belong to another network. they can communicate with eachother
fine through the switch. What i need now is to get the information from
their COM ports and allocate an IP address for each PC so that the
information can be switched and routed, and also received from a remote
network. I have been trying to configure the xyplex to do this for a
long time now and i simply can't manage it. I have enabled PPP by using
define server protocol PPP enable, i have set up an ip address for the
terminal server, along with a valid gateway. I can ping the terminal
server and vica versa from the computers and switch, when the terminal
server is connected to a port in the switch. Can someone explain how
the xyplex must be configured please?

Thanks for your time
Michael