What avionics systems are interfaced with ACARS?
ACARS, otherwise known as Aircraft Communications and Addressing Reporting System, is almost always interfaced with the #3 VHF Communication System. Ordinarily, that #3 VHF system is set for "Data Mode." Some aircraft have the ability to use the #3 VHF as an auxiliary communications system by putting it in voice mode, and other aircraft dedicate it to strictly data mode. 

However, I have seen operators tie the ACARS system to the #2 VHF system when there is no #3 VHF system installed. In this case, it's ordinarily up to the pilots to switch between voice and data mode on the #2 VHF system. I can only imagine how annoying that must be!

While VHF is most frequently used on domestic flights, its relatively short range does little good on international flights over water. On aircraft meant for such long hauls, the ACARS system is usually tied to the HF radios and the SATCOM system. If I recall correctly, the ACARS system opts for VHF communications as the primary method. If there is no VHF available, or its rendered inoperative, the secondary method is often SATCOM; failing that, the HF systems. 

In order for the aircrew to interface with the ACARS system, there is some sort of display with a keypad they interface with. On the vast majority of aircraft I've worked on, that display is often referred to as either the MCDU or CDU. On aircraft with MCDU's (Multifunction Control Display Unit), the pilot can select the mode they prefer to be in (ACARS, FMS, etc...), and can interface with the appropriate system through that singular central display - kind of like selecting what program you'd like to run on your computer. If the pilot selects ACARS, they can interface with all the relevant data pertinent to the ACARS system. If they select FMS, they can interface with the FMS (Flight Management System); and so on and so forth. 

On aircraft equipped with CDU's (Control Display Units), usually there are completely separate displays/interfaces for the FMS and ACARS. In that case, there are a plethora of options from a number of different manufacturers. I've seen touch screens, a secondary but different style CDU dedicated to ACARS communications, and so on and so forth. It's wide and varied. 

The MCDU and/or CDU usually interfaces with an ACARS Management Unit. That management unit is ordinarily the "brain" of the ACARS system. All pilot interactions with the MCDU/CDU are sent to the ACARS system, usually via a 429 Databus. The best way to think of this is that the MCDU/CDU setup is like your computer monitor and keyboard. You input data and see those inputs with keyboard and monitor, but the real "action" is taking place with the memory and processor "behind the scenes." That behind the scenes action is the ACARS Management Unit.

The ACARS Management Unit (MU) can then interface with a variety of other systems and subsystems largely based on operator configuration. Some operators choose to log crew hours with the use of OOOI (Out, Off, On, In) times based on subsystem inputs to the ACARS MU. For example, when the beacon turns on, an input to the ACARS MU generates the "Out" time. When the aircraft takes off, the ground sensing mechanism of the aircraft sends a separate signal to the MU letting it know that the aircraft has left the ground. When that ground sensing mechanism changes state again (aircraft lands), it sends another signal to the MU letting it know that the aircraft has landed. Finally, when one of the doors (usually the L1 door) is opened after flight, the ACARS receives a signal from the door switch, and the "In" time is generated. 

Sometimes the MU is tied to an ACARS Aircraft Personality Module (APM). This is usually just a separate unit with DIP switches set to a certain configuration so no matter what MU is installed on a particular aircraft, it can pick up the "correct identity" and act accordingly. Other times, there is no APM and the configuration control is handled exclusively by the software load often verified by the mechanic after installation. 

Also, with the advent and implementation of ADS-B aircraft, GPS position data has begun to tie in to the ACARS system. If you want to see a mechanic roll his eyes, ask him to do a full functional check of the ADS-B system in the rain or cold. It's hours long, it never fails, and the consequences of what happens if it is failed are negligible. Most of us see it as a giant waste of time... But I digress.

Finally, I'd like to note that all of this information is with the caveat that every operator is different, and so is every aircraft. There are endless combinations of systems that could or could not be tied into the ACARS system. Generally speaking, though, what I've mentioned above is a fairly standard installation.