How many devices we can scan

[UnitedBus]

hi support

Thinking of attendance system using bluetooth Identity cards

what is maximum nos of cards can a nrf52  scan at a time.

suppose there are 100people in a room , can a device scan all 100 or is there any limitations

thx

[Andreas]

Hi,

 

A nRF52 can only receive one packet at a time, mean it can only scan one card at a time. In order to scan e.g. 100 cards, it would have to scan them sequentially.

 

Best regards,

Andreas

[UnitedBus]

Then it is possible to scan Card No 1  twice before it scans Card No N  and Card N + 1 may remain Unscaned Also Right 

Is there any way to guarantee all the cards are scanned ?

[Andreas]

Hi,

 

Yes, it is possible that cards will be scanned multiple times before others are scanned once, as long as the cards that are scanned keep on advertising. This will probably be exacerbated by the position of the people in the room, the ones closest to the scanner will 'win' each time in the case of a conflict, and I would not expect people to move about all that much between advertising packets (max ~10 ms)

 

The first thing that comes to mind is to in your application try to make a card know when it is discovered and then cease to advertise for sometime longer than the advertising intervals. You could possibly try to use the a scan response for this. Combined with a not-too-short advertising interval this should reduce the amount of congestion and be possible to fine-tune to some extent. A higher interval would reduce the likelyhood of congestion, but add some 'lag' to the system, but again there might not be a need for getting an update from every card all too often.

 

Also, I would assume that the 100 persons do not suddenly appear in the room out of thin air, they would be entering at some rate over some time, allowing for this method to take effect gradually.

 

Best regards,

Andreas