if (incomingByte == 'th') {
Single quotes denote a single byte. th or tl aren't single bytes, they are strings. When you read from Serial, you need to store single bytes in an array and use something like strcmp() to compare the strings.
if (incomingByte == 'th') {
Single quotes denote a single byte. th or tl aren't single bytes, they are strings. When you read from Serial, you need to store single bytes in an array and use something like strcmp() to compare the strings.