CH340 is a USB converter chip, which converts USB to serial UART interface or to printer interface. In UART mode, CH340 provides common MODEM interface signals, to expand UART interface of computer or upgrade common serial communication devices to USB directly. For more information about converting USB to printer interface, please refer to the datasheet CH340DS2.
Model differences: CH340C, CH340N, CH340K, CH340E and CH340B integrate 12MHz clock, no external crystal required. CH340B integrates EEPROM used to configure the serial number, etc. Some functions can be customized. The CH340K has three diodes built in to reduce current flow backwards between the I/O pins of the MCU. The backplane of the CH340K is 0# pin GND, which is an optional connection; the 3# pin GND is the necessary connection. CH340R provides reverse polarity TXD and MODEM interface signals. (Discontinued).