Dealing with unicode �, how to get rid of? Android/java

I am using a terminal emulator library to create a terminal and then I use it to send the data entered over serial to a serial device. The library can be seen here.

When I enter data into the terminal a strange series of characters is being sent/received. I think the unicode replacement character gets sent over serial, the serial device doesnt know what it is and returns ~0.

Screenshot of what appears in the terminal when i write "test":

And the log showing the strings sent and the data received.

I create an EmulatorView, it's the terminal view. it mentions the diamonds here.

private void sendText(CharSequence text) {
                int n = text.length();
                char c;
                try {
                    for(int i = 0; i < n; i++) {
                        c = text.charAt(i);
                        if (Character.isHighSurrogate(c)) {
                            int codePoint;
                            if (++i < n) {
                                codePoint = Character.toCodePoint(c, text.charAt(i));
                            } else {
                                // Unicode Replacement Glyph, aka white question mark in black diamond.
                                codePoint = '\ufffd';
                            }
                            mapAndSend(codePoint);
                        } else {
                            mapAndSend(c);
                        }
                    }
                } catch (IOExceptio