Hmm, well it seems possible to detect both open and closed circuit on the NTC. Because I have just tested here on the original FW, and either way, the invalid ADC input value is noticed as out of range, and the "room temp" on the main screen then prints as text: "! 14°C"
Perhaps a sensible compromise would be to think in an extendable fashion. ATM we have 2 settings for the tip choice:
* T12
* T245
However there is no NTC present with T245 handle. So you don't need to display warning message at boot time. Unless you also detect the invalid NTC value too during boot time. Which would then suggest *maybe* the JBC tip. Or *maybe* bad wiring of the T12 handle. Then ask the user to positively manually select between T245 and JBC. So then display the warning message.
Otherwise it should be safe assume (most cases), if you get a valid NTC data it must be because a T12 handle is already connected to the station. Before the user powered on.
But in future, the planning of 1 additional option, is also possible. So then a 3rd option can later appears:
* T12
* T245
* JBC Auto
With "JBC Auto" being the auto sensing option. In this case, the user must manually choose to enable this option, by going into the menu settings after boot. However if somebody chooses this 3rd option, then they are no longer protected from faulty T12 handle wiring, giving a faulty NTC reading. This is then a good idea to display a 2nd warning screen, after the user selects "JBC Auto". To warn them of that.
An open circuit on NTC (--> 3v3 NTC adc max) should set station to --> T245 handle. However in 3rd planning stages. An additional mod board, hardware mod is installed. Which is to say for future expansion. Then a closed circuit on NTC (--> 0v NTC adc min). Would set the station to be T210.
I think that is the best way to map future expansion, to select multiple tips. Simply by physically plugging them in. The auto detect feature is useful when you frequently want to change handles.
For example, if you have cheap T12 tips for some shapes. Then more expensive JBC tips for other shape. Also the T245 has larger size, for large work. And T210 is smaller, better for fine micro soldering.
I think that is really the only way I can see ATM. To keep firmware compatibility, and make an extensible system. For both modding, and not modding, and new hardware.
Otherwise if you don't use the NTC value alone. Then it can be supplemented with additional extra GPIO digital IO pins. But that as you say original boards, which don't break out the pads for not-used pins. So then they are left to micro soldering some tiny fly wire (mod wire) and glueing it down with epoxy. Onto the legs of the MCU. Which is not always so easy, for the initial capabilities of beginner / novice user.
But anyhow, it's just something you can start doing right now! Check NTC value is valid at boot time. If it's valid it cannot be any JBC tips. ---> must be T12. --> No need to display the warning screen at boot up.
Similarly, after boot time: If T12 handle is the selected option. Then constantly be checking that the NTC is a valid value (not ADC max, or ADC min). If that ever happens, then stop all power to the heater. And display an error message.