Pete,

This can get kind of involved.  We have videos on communications that should help with this. 

Depending on your processor, there are multiple ways to connect to the PLC. Let's assume that you are connecting via RS232/DF1 for discussion purposes.  When first communicating with a processor, I start off by making sure I have a good comm link by going to the COMMS menu option and selecting system comms.  that will bring up a RSwho screen that will show all your configured drivers an what processors are actively communicating with those drivers.   If your driver is configured and you see an active connection, you can click on the node and download your program from there.   You should be able to go  online from there.  Once online, save your program and you should not have the connection problem again. 

If you have not configured a driver in RSLinx, you need to go to rslinx, configure driver (for our example the RS232/DF1).  Go to RSWho in rslinx to insure that it is working and then follow the instructions above.   It may take a couple posts to figure out your problem so post back if you have additional issues.