You are attempting to talk to your peripheral via a second serial UART implemented in software on arbitrary pins, which should leave the hardware UART free for uploading and debug or other runtime communication between the Arduino and a PC. However, the 'arbitrary pins' you choose to use to connect the peripheral are the same pins as used by the hardware UART. Move your peripheral to two other pins, for example int RX_PIN = 2; int TX_PIN = 3; and move the connections accordingly Also consider if you really want Serial.println() which will add a line break after every character in the output, and will also print the received characters as their numeric ASCII codes, one per line, rather than printing the characters themselves. You probably want Serial.write().

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