you know how to create a webservice.

your legacy app is already working, I suppose.

it sounds to me, that what you need to do is build an interface between
them.

There are lots of ways to do this,   among them.
 shared memory
 local sockets communication
 message queues (like MSMQ)
 shared database
 local filesystem (a shared file)
 webservices

You did n't say what the legacy system looks like, or how it is deployed, or
what existing interfaces it exposes.  You said it reads a serial port.  What
does it do then?   what else does it read or write?  How does it make its
results known to the world?

Is there a network interface to it today?  does it "listen" on a queue, a
local file, a socket - for incoming requests or commands?  Maybe it exposes
a WMI interface for administrative control.   If there is an existing
interface, the next question is, can the new webservice code utilize it, or
could the new code utilize maybe an extended version of that existing
interface?

example:  Does the legacy system, today, read and write to a database?  If
so the webservice could connect to the same db.  What about a message queue?
Likewise.  etc etc

You could even use a webservice call from the webservice to the legacy
system.  Supposing the legacy system is implemented in C++, there are
webservice toolkits for C++, you could extend the legacy app to expose this
new interface. Likewise for other languages.

-D