I have purchased an RFID reader that communicates using an RS-232 cable. I have however used an RS-232 to USB adapter to connect it to my laptop. It is an ACM812A RFID reader.
When I open the RFID reader's application and try to connect to the reader it says communication error. So I decided to pull out the Arduino and start seeing what is being transmitted. First, every time an RFID tag is placed near the RFID reader, the reader beeps to say a tag has been detected. The tag's ID is then transmitted via the RS-232 cable to the PC. So I decided to remove the RS-232 cable and connection. I then took three wires and connected it to the reader's RX, TX and GND pins and then connected the RX to TX of the Arduino and TX to RX of the Arduino and also ground.
When I open the serial monitor and when I place the tag near the reader I get the following displayed on the serial monitor for all buadrates. I was supposed to be getting the ID from the tag (like 251458712). When I measured the TX wire of the reader, it is -5.2V. Can someone help me to understand what is going wrong?