I know that was some discussion on this and some people said that mixing firmware was a problem, even with 5.5.10. I hadn't seen the problem until about a week ago. Had PowerBridge talking to a Powerbeam talking to a PowerBridge which then link to a another PTP using NS5's and an AP on that end. It seemed to work great for several months. A week ago, the AP at the end started pushing through 10-20 pings and then dropping 3 pings. After finding nothing, we pull the PowerBridge and replaced it with a PowerBeam. Problem solved.
On the other hand, we have a boatload of NS5M's, some with XW firmware connected to XM Rockets. No problems so far. Rory
