EXCELLENT call, westaust55! I had not noticed the external diode requirement previously.
All 8 and 14 pin chips: Due to the internal structure of input3 (C.3) on these chips, a 1N4148 diode is required between the pin and V+ for serin to work on this particular pin ('bar' end of diode to V+) with this circuit. All other pins have an internal diode.
All 20 pin chips: Due to the internal structure of input6 (C.6) on this chip, a 1N4148 diode is required between the pin and V+ for serin to work on this particular pin ('bar' end of diode to V+) with this circuit. All other pins have an internal diode.