Hi RonnieG,
I had two projects that req'd SDLC to talk to instruments.
In both cases we located a suitalbe hardware provider that included a dll for their device.
Appropriate wrappers were developed to use the supllied dll's.
SDLC adapters are also available from companies like Black Box provided the transfer rate is low enough.
In my applications, I need to be able to handle some non-standard baud rates on multiple channels operating at high rates, so we went with the board based solution in both cases to handle the flexibility and performance issues.
I would suggest you talk to the people who are going to be supporting the dll before you buy. You want to have a resource that will not "go critical mass" when you mention LV.
I hope this helps,
Ben