Yes, the INPUT$(1) means to read from COMM port #1.
Note that INPUT$(n) command is non-blocking, meaning that it WILL NOT wait there until a string become available, (otherwise the rest of the ladder program will cease to work, which is not desirable).
A string is only accepted when terminated with a CR (ASCII 13 or 0D Hex) character. Hence the PLC's serial port will accumulate all the characters until it sees a CR character and then it will past the entire string to the INPUT$(n) command.
If there is no string availalble, INPUT$(n) returns an empty string. Hence you need to test whether the return string is an empty string. The easiest way is to check the length of the return string:
e.g. A$ = INPUT$(1)
IF LEN(A$) <> 0 ' a string is available
.....
ENDIF
You can use a clock pulse to periodically execute a custom function to check for available string. Incoming serial characters are automatically stored in system buffer up to 256 characters so they won't be lost even if you don't check the serial port very regularly. A one second clock pulse maybe good enough for this purpose.