: Negative voltages between -3V and -15V.
Understanding the pinout is essential for using this module. The board clearly labels its connection points. The 4-pin header is for your microcontroller, and the DB9 connector is for your RS232 device. hw-044 datasheet
For typical Arduino connections, connect the Arduino RX pin to the module's RXD pin : Negative voltages between -3V and -15V
Ensure the baud rate matches the device on both sides. hw-044 datasheet
Even with a complete datasheet, issues can arise. Here are the most common problems and solutions.
Connects to the Negative (-) terminal of the single-cell Lithium battery.
Ensure a 0.1µF ceramic decoupling capacitor is placed close to the VCC and GND pins of the module to filter out power supply noise.