The iPhone6/6s/6+ are NOT designed to read passive NFC tags (aka Discovery Mode). There's a lot of misinformation on this topic, so I thought to provide some tangible info for developers to consider. The lack of NFC tag read support is not because of software but because of hardware. To understand why, you need to understand how NFC works. NFC works by way of Load Modulation. That means that the interrogator (PCD) emits a carrier magnetic field that energizes the passive target (PICC). With the potential generated by this carrier field, the target then is able to demodulate data coming from the interrogator and respond by modulating data over top of this very same field. The key here is that the target never creates a field of its own.
If you look at the iPhone6 teardown and parts list you will see the presence of a very small NFC loop antenna as well as the use of the AS3923 booster IC. This design was intended for custom microSD or SIM cards to enable mobile phones of old to do payments. This is the type of application where the mobile phone presents a Card Emulated credential to a high power contactless POS terminal. The POS terminal acts as the reader, energizing the iPhone6 with help from the AS3923 chip. The AS3923 block diagram clearly shows how the RX and TX modulation is boosted from a signal presented by a reader device. In other words the iPhone6 is not meant to provide a field, only to react to one. That's why it's design is only meant for NFC Card Emulation and perhaps Peer-2-Peer, but definitely not tag Discovery.
There are some alternatives to achieving tag Discovery with an iPhone6 using HW accessories. I talk about these integrations and how developers can architect solutions in this blog post. Our low power reader designs open interesting opportunities for mobile engagement that few developers are thinking about.
Disclosure: I'm the founder of Flomio, Inc., a TechStars company that delivers proximity ID hardware, software, and services for applications ranging from access control to payments.
Update: This rumor, if true, would open up the possibility for the iPhone to practically support NFC tag Discovery mode. An all glass design would not interfere with the NFC antenna as does the metal back of the current iPhone. We've attempted this design approach --albeit with cheaper materials-- on some of our custom reader designs with success so looking forward to this improvement.
Update: iOS11 has announced support for "NFC reader mode" for iPhone7/7+. Details here. API only supports reading NDEF messages (no ISO7816 APDUs) while an app is in the foreground (no background detection). Due out in the Fall, 2017... check the screenshot from WWDC keynote:
