Quote (Admin)
1. If use only TXD, RXD GND three pins to communicate, RTS Pin connect to GND directly. DTR Pin is pulled up inside. If unused keep pins open. Даташит SIM300D, страница 19, качаем ЗДЕСЬ
Стр 36: Note: DTR Pin should be held low level during communicating between the module and DTE.
Поэтому и непонятно.