Don't forget how much capacitance a transzorb will add to your high speed data lines! Because it's a heavily doped junction to make it break down at a fairly low voltage, and a big junction to cope with the surge, it's at least 1nF.
For a pair of data lines I add a bridge rectifier (such as MB6S) with the signal on the AC inputs, and I connect the - terminal to ground* or the +terminal to the transzorb. Then I bias the transzorb so that it is slightly conducting (a fraction of a milliamp).That keeps all the diodes in the bridge reverse biassed. The bridge diodes have a capacitance of 13pF, so the data lines are isolated from the capacitance of the transzorb (and you only need one transzorb for any number of data lines). AS the bridge diodes are never forward biassed, their reverse recovery time is not a problem.

Don't be tempted to clamp the signal to your 5V supply with a diode. Your decoupling could absorb a small surge, but when some idiot connects 24V DC to it, instead of just blowing up the line driver/receiver, he can destroy a whole board of logic.

* for RS485, connect to a 5V transzorb, as the RS485 signal can be between -5V and +12V.

 

Standard Zener diodes are not used generally for digital IO,
there capacitance is generaly very high , which along with the resistance needed to stop the over current, kills any edge speed,

There are proper ESD protectors, e.g.
https://www.nexperia.com/products/e...LTERING/#/p=1,s=0,f=,c=,rpp=,fs=0,sc=,so=,es=

but note these are aimed at protecting against static / spark voltages, which are typically modelled as having a significate series resistance , such as the human body model.

https://www.esda.org/esd-overview/e...eters using parametric and functional testing.

Protecting power inputs, again , zenars have limited usage.
A zenar will limit the voltage going to the circuit, but if you drawing say 3 amps, that should be at 3v3, so you put a 4v zenar in, and you supply it with 5 volts, then your dropping 3 watts across the zenar. Zeanrs tend to be used when there is a fuse function in the circuit, even then, crow bars are more likely, that clamp the input to zero and protect the input.