Is it possible to send an ASCII code on an external device through USB port using IOS API? I assume that it is possible through bluetooth connection, but I'm not sure through USB connector. Any thoughts?
I would appreciate if someone could put me in the right direction.
Thanking you in advance
That largely depends on the level of sophistication of this "external device". If this external device is just a bare metal embedded device without some sort of operating system you definitely need MFI.
Otherwise, you can run a usbmuxd server (e.g. on Raspberry PI with Linux) to establish a connection via USB. Usbmuxd is basically the technology that Apple uses to communicate between iOS devices and Desktop apps such as iTunes or Xcode. And yes, it is App Store compliant, e.g., Duet Display uses this approach to make an iPad a secondary display for your Desktop via USB.
There are several open source libraries that provide a high level API, e.g., PeerTalk or DarkLightning.