Electronic – How to identify unknown component

I'm going to post this as an answer as I've seen the same type of question come up multiple times, and for each I've basically commented the same thing:

It's pretty much impossible to identify this sort of component unless you either have a BOM of the circuit, schematic, or already know what they are.

If you have some rough idea of the function they perform and it is something more complex than say a discreet transistor, then you may be able to identify them by searching for parts with that functionality on manufacturer websites and comparing marking codes from datasheets. But really this is a shot in the dark.

Having said that, based on your description, the SOT23 package device is almost certainly some sort of TVS device - used for ESD and Transient (EFT) protection on inputs. These are quite common on RS485 devices as per this TI application note. There are components like this specifically for RS485 in the same package like this one. Doesn't have the same marking code, but will like have the same function.

The other package as pointed out by @jp314 in the comments is likely some kind of line filter. As you have found WE has some in a similar package with similar markings, so this is quite clearly what it is. In fact looking at the datasheets from that link, this part has a marking of 500 and looks identical to your picture.

For both of these parts, it could be inferred from the circuit what the parts do. Finding a datasheet for the exact part is the hard bit.

OS stands for "Oscillator" in this case.

The pictured component OS1 is a 3-terminal ceramic resonator, such as this:

^{Image Source}

With the marking 400C it is probably a 4MHz frequency like the one pictured. This is a fairly standard frequency for cheap MCUs. The resonator will provide a clock to the microcontroller.

These resonators are very cheap parts which do not provide accurate timing (±1% is a typical accuracy). However in this application the frequency of the MCU is probably not all that important.

OS2 is a 32.768kHz crystal oscillator.

These are low frequency high accuracy crystals which are typically used for creating an accurate 1Hz (1 second) clock signal. The reason for the very precise frequency is that it is a power of 2 (2¹⁵ to be exact) which makes dividing the frequency down to 1Hz very easy (using a simple free-running binary counter).

Additionally the circuitry required to run these low frequency crystals to get a 1Hz is very compact and low power, which makes it ideal for running off a battery like you would have in a watch.

In this example there is a small battery in the centre-left of the top image. When not supplied with an external power source, the rest of the circuitry will be powered down but the clock oscillator will keep going to keep track of what time it is.