Bluetooth PAN

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I have built some modules that are housed together in a rackmount arrangeme
nt.
Next I want to build a master device (in same rack) to monitor the state of
 each of these devices.
Hardwire is not an option.
Bluetooth (or Zigbee) seem to be best options.
Generally, there are <20 of these modules in a rack but there could be hund
reds.

Slave modules would report their state to the master. Master simply logs sl
ave ID, state and time. - that's the easy part.

The difficult part is that these modules will turn on and off independently
. Some may be nearly constantly on, turning off for only a few seconds whil
e for others, the reverse may be true. In most cases, it is the time they t
urn on and off that I am most interested in. I should have enough voltage i
n the module power supply after power is removed to get out one last (goodb
ye) message to the master.
The other problem is that several slave modules may turn on or off nearly s
imultaneously. Not overly worried about time resolution for the logging.
From what I gather, once the slave is powered off it will need to be paired
 again with the master when it turns on. I'm concerned about the time it ta
kes for the master to find and pair with the slave.

Any way around this?
Any ideas?

thanks

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