Isolated or Not?

Ok, I have an issue, we're busy planning some corrective measures to move everything to -48v power on the sites, as that's what our batteries/rectifiers provide and off of our old 110v.

Now I already know that the CMM does NOT like -48v for the 56v port as apparently the way the cmm3/4 are designed they ground to chassis which when you reverse polarity on -48 to +48 by swapping cables it grounds out the entire cmm and causes huge issues. 

No problem we're going to get Isolated -48 to +56v dc-dc converters for the CMM and 450's and an Isolated -48 to +30v also for the cmm and our old canopy 100's.

But we also have sites with a number of ptp500/600, mimosa's, etc. We're using PIDU PLUS's for the 500/600 and even mimosa's and they seem to work great on 110v and they even seem to work on -48v sourced power... (i guess swapped polarity), but I wanted to confirm with cambium and the other users, is this ok? can we power the PIDU Plus and rely on it to isolate properly the -48v to +48v power? Or do we need to use a -48 to +56v isolated dc-dc for those too?


I can provide some information about PTP 500, PTP 600 and the PIDU Plus. These are all isolated designs, and there is no direct DC connection between the PoE port and the chassis. The ideal way to power these products from DC is to connect a floating 56 V supply to the DC input terminals on the PIDU Plus. The system will work satisfactorily with a ground-referenced negative supply, but lightning protection is not as good as in the case with a floating supply.

If the DC supply is really 48 V (not 56 V) then the maximum cable length for PTP 500 and PTP 600 will be reduced below the standard 100 m. Often "48 V" supplies are set at a higher voltage, similar to a nominal 48 V lead acid battery on charge.

There is no DC/DC converter in the PIDU Plus, and the DC input connects directly to the drop cable through transformer windings.