The CH340G is an inexpensive USB serial adapter chip - similar to the better known FTDI FT232 chip. Programming an Arduino Pro Mini, a breadboard Arduino, or one of my ATtiny boards requires a USB serial adapter with a 6-pin header with GND, Vcc, Tx, Rx, and DTR. This has traditionally taken the form of an "FTDI" adapter using the FT232 chip, however, not only are these parts more expensive, but they are frequently counterfeited, and the counterfeit chips do not work with the new drivers (Google "ftdigate" for the full story). The CH340G serial adapters are reliable and are not known to have a counterfeiting problem.

This CH340G-based adapter not only has the usual "FTDI" 6-pin pinout, it also breaks out the five other "modem control" pins - RTS, CTS, DCD, RI, and DSR. RTS is an output, the other four are inputs (gently pulled up, active low). Full featured terminal software, such as hTerm (free) allows the state of these pins to be viewed on the screen, and the RTS and DTR pins to be asserted and deasserted by clicking the corresponding buttons. When connected to a typical Arduino board, twiddling DTR will reset the board - RTS is unused. This functionality can be a great help in debugging.
In the 6-pin "FTDI"-style pinout, the second pin, normally CTS, is left open, as most Arduino boards that use that header simply tie that pin to ground, which would render CTS useless.

Features

• 500mA self-resetting PTC fuse
• 6-pin programming header for Arduino Pro Mini et. al.
• Mini-USB connector (I have not been satisfied with the durability of Micro-USB)
• Tx, Rx, and Power LEDs in the form of an RGB LED. Series resistors are 2.2k on tx and rx
• All modem control pins broken out
• Available in 3.3v and 5v versions
• 3.3v version uses a REAL VOLTAGE REGULATOR, not two cheap signal diodes in series (unfortunately common in low-end serial adapters).
• Assembled in the USA.

This board is sold ASSEMBLED as shown, WITHOUT pin header.