Ivanhr use 'are' regardless of 'a plurality' being singular.
I
I don't see it that way. Plurality - despite its meaning, is singular in form.
"A plurality of devices is connected.." - is a badly formed sentence in my opinion. If one intends to mean more than one device, I sugges "multiple", which is a better choice of word.
"A plurality of devices are connected" is not grammatically correct because the true subject of the sentence is singular.
“A plurality of devices is connected” is grammatically correct, but it is not logical because a plurality cannot be connected to itself. It is the devices that make up the plurality that are actually connected to each other.