All notes
Hardwar

Basics

Fast charge ipad 5v2.1a, iphone 5v1a, usb 5v600ma.

Transmission

RS-232, RS-485

Difference between RS-232 and RS-485.

RS-232

One of the oldest serial interfaces is generically called RS-232. It was originally established in 1962 as a method of connecting data terminal equipment (DTE) such as electromechanical teletypewriters to data communications equipment (DCE).
The standard defines a logic 1 and a voltage between -3 and -25 V and a logic 0 as a voltage level between +3 and + 25 V. Voltages between ±3 V are invalid, providing a huge noise margin for the interface. In common practice, logic 0 and 1 levels are typically as low as ±5 V and as high as ±12 or ±15 V.
The official name of the standard is Electronic Industries Association/Telecommunications Industries Association EIA/TIA-232-F. The letter F designates the most recent standard modification and update.
Many RS-232 connections are one-way or simplex. However, using the special signaling and control voltages available, two-way or half-duplex operation is possible. The two connected devices alternate transmitting and receiving operations.

RS-485

Also defined by the EIA/TIA standard, this interface is now called TIA-485. It defines not only a single device-to-device interface but also a communications bus that can be used to form simple networks of multiple devices. Its configuration and specifications also extend the range and data rate beyond the RS-232 interface capabilities.

Comparison

In general, the RS-232 is best for short-distance low-speed requirements. It is simple and low cost, and plenty of components like line drivers and receivers, UARTs, and connectors are available to build the interface.

The RS-485 is for higher speeds over longer ranges or if duplex networking capability is required. Again, many standard parts are available to create the interface.