petfy:
The UPS data sheet is not specific on what a high digital signal is (in volts).
Then you have to measure it yourself.
Please post a link to the data sheet.
petfy:
Opto-isolators only seem to be used for noisy signals.
What makes you think that? They are used when galvanic isolation between two devices is required. The UPS and the WeMos can be at different potentials, they can use different signal voltages, and if for some reason the signal voltage of the UPS gets too high, it will blow up the opto-isolator, not the WeMos.
petfy:
As far as I can research some, if not, most of the digital pins have an intrinsic function attached to them.
That doesn't matter. A digital GPIO pin is just that: a digital GPIO pin. While it is true that you can disconnect some pins from the GPIO registers and connect them to hardware peripherals (SPI, UART, I²S, ...) instead, that doesn't matter if you don't want to use these peripherals.
petfy:
D2 and D1 are the SDA (serial data) and SLC (serial clock) of the I2C bus.
No, the ESP8266 doesn't have a hardware TWI (a hardware peripheral that's used for I²C), so it is bit-banged in software using normal GPIO pins. You can use I²C on any two pins. (But again, it doesn't matter if you're not going to use I²C.)
If you don't need SPI, don't enable it, and the "SPI" pins will just be normal GPIO pins.
If you're going to use D4 for an LED, select different pins for the signal inputs, obviously.
You have to keep in mind that GPIO0 and GPIO2 should be high at boot time, GPIO15 should be low at boot time. GPIO0 has a pull-up resistor, GPIO15 has a pull-down resistor. GPIO2 has an internal pull-up resistor that's enabled at boot time, but it probably has an LED connected to it.
https://tttapa.github.io/ESP8266/Chap04%20-%20Microcontroller.html
petfy:
Furthermore, I assume I connect alarm signal return to the GND pin?
Measure the polarity first. Return seems to imply that it sinks current, so that it's indeed ground, but you have to check it.
Pieter