Here is a schematic of the current setup.
Not clear why you need more than one RFID per locker?
I don't. Just the one RFID per locker. 10 x lockers per Arduino = 10 RFIDs per Arduino. These are assembled into a modular package which can then be plugged into a FoxG20 via a USB hub. I want to design the system in such a way that a faulty module can simply be unplugged and replaced. One of the modules will be able to control an eleventh RFID reader and an alphanumeric LCD, to be mounted externally on the locker unit, to act as a card reader and display.
I was merely trying to justify my design choices by explaining a little more of where I want the project to go. The current system is just a single locker, I am testing the set up before adding complexity.
Thats a problem here is that we don't see the whole picture so we can't tell what your design objectives or tradeoff decisions are. How far away from each other are the lockers? Are they metal or something that can form a natural Faraday shield to protect them from interference from other RFID receivers?
I apologise- I didn't explain everything in the first post as I feared it would have been a short novel and no one would have read it. I have explained my current set up, problem and supplied a schematic.
The lockers are next to each other, big enough to knee down inside, they are fully enclosed and metal, and I believe provide a perfectly adequate natural Faraday shield.
What is the startup time for an RFID reader? It is practical to "multiplex" them?
I take it the startup time is how long it takes for the antenna to charge ... in which case I'm not sure, but I have programmed the Arduino to keep the RFID powered for 3 seconds. In my test set up (Fox removed from circuit, Arduino plugged into my laptop) the RFID scanner charges, reads the tag and transmits it to the Arduino in the blink of an eye, and will read tags upwards of 10cm. With the Fox connected the same process can take a few seconds, will only read tags closer than 2cm and sometimes doesn't read them at all.
Do you think it is likely to be RF interference from the Fox, passed through the USB cable? Or could the Fox be reducing the current flow to the RFID reader?
Using multiple Arduinos and USB and hubs just seems unnecessarily complex and overkill. Extra development complexity, extra component cost and no apparent system benefit? Unless there is some other factor you didn't mention here
It's purely so I can separate the system into modules. For instance, if 10 more lockers are added to the unit at a later date I can simple plug another module into the Fox rather than rework everything. In any case, the problem is being caused when I add the Fox to the circuit. Discussing whether I should omit the Arduino is not going to solve that.
It is practical to "multiplex" them?
I have tested using multiple RFID readers muxed through a 74HC151N into a single Arduino Rx, which works fine, but my system requires RFID readers for two different functions. There are the readers inside each locker used to monitor contents- they don't have to be on all the time so so muxing is practical. There is an external card reader however, providing user access to the locker unit, which should be powered up all the time. I couldn't get two RFID readers to work at the same time on the same circuit, so resigned to powering down the card reader when a locker reading was required to scan for contents.