iOS has no API for accessing generic USB peripherals. Some USB peripherals are managed by iOS and made accessible to our app via high-level APIs — for example, events from a USB keyboard are routed to apps via the UIKit event system, including low-level events via
UIPress — but that’s not the case for USB serial devices.
If you control the hardware then you can update it to support
MFi and make it available to your app via the ExternalAccessory framework.
If you don’t control the hardware, your options are much more limited. For prototyping and limited deployment, you may be able to get away with a Redpark cable [1]. If you require widespread deployment, about your only option is to interpose some hardware that is directly accessible from iOS. For example, you could buy or build a serial-to-Core Bluetooth bridge.
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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@apple.com"[1] For example, their Lightning Serial Cable (L2-DB9V3) <redpark.com/lightning-serial-cable-l2-db9v/>.