Input Protection Circuits – Using Diodes Effectively

Under normal operation, the input voltage of an LT3015 should always be more negative than the output voltage, e.g. -12V in, -5V out. What the datasheet is saying is that if the output is somehow driven 0.3V lower than the input, e.g. -6V at the output, -5.7V at the input, then a parasitic diode that's part of chip substrate may/will start conducting. You would then have current flowing in an unexpected way, and this may damage your regulator or other connected circuitry. For example, if you hooked the regulator up like you would a positive voltage regulator, then this:

^{simulate this circuit – Schematic created using CircuitLab}

effectively becomes this:

^{simulate this circuit}

Under these conditions, your regulator has a good chance of functioning as an short-lived electric heater, releasing smoke or otherwise failing. That said, if you keep your polarities correct, this is unlikely to happen otherwise.

When the datasheet is talking about reverse input/output voltage protection, it means that the output can be ABOVE ground without damaging the regulator, i.e. "If IN is left open circuit or grounded, OUT can be pulled above GND by 30V"; "In dual supply applications where the regulator’s load is returned to a positive supply, OUT can be pulled above GND by 30V and still allow the LT3015 to start up and operate." That sort of configuration would look like this.

^{simulate this circuit}

While both supplies are connected, the regulator provides regulation, current protection and thermal protection. If V2 is disconnected, the regulator will safely stop conducting, and if the regulator is shut down, the OUT pin can rise above ground without damage, and the regulator can later be turned on (without disconnecting power supplies). The important thing is that the IN pin should always be more negative than the OUT, ADJ, and SHDN pins to keep current from flowing through the parasitic diode. If you're concerned about protecting the regulator from reverse polarity, a diode in series with your power supply should suffice, though remember that the diode will increase the dropout voltage by ~1V.

Your requirements make it a difficult to design a circuit. And I think you'll have to compromise somewhere.
I have yet to encounter an IO port like you describe on something industrial. Which is what you're asking with the 24V rating.

But, let's define the requirements:

1. Pull-to-ground Digital Input.
2. Digital 24Vdc input (eg: IEC1131-2)
3. High Frequency 24V counter.
4. TTL level UART.

But let's add:

5. Noise immunity, you don't want the input toggling at mains frequency due to noise or nearby events.

Now, this provides some serious challenges, the ranged your input threshold are <0.8V and >2.0V, but also <5V and >15V.
To add complications, the 24V input will need to sink some (~3mA) current, while the uart will need to pull-up the line since the transmitter pulls it down.

I think you can't make this without some selector switches.

^{simulate this circuit – Schematic created using CircuitLab}

R1 should be low enough to allow uart to pass. R2 should provide enough current if the uart transmitter is not push-pull (they often are not). If not, lower R1 and R2, but keep an eye on the current and power. They're doing 50 mW already.
R3 should take enough current for 24 Volt to not excite the zener diode, and provide noise immunity since it requires current to get above the buffers threshold voltage.
C1 will provide some filtering for fast glitches, but won't be very effective. Since it can't slow down the signal below 10 kHz (uart baud rate).

The buffer is there as sacrificial component, or as component with stricter thresholds, or threshold with less spread between parts.
Schmitt trigger inputs are strongly recommended.

The compromise here is that a pull-to-ground might have a slow recovery due to R2. But you can't lower R2 much, since the low-level thresholds of will be a problem.

Simulations are great, but I would definitely put it on a breadboard before I order the PCB. Especially the UART part is tricky.