Request for advice with a DIY Project (TTL)

[Maxtor]

I have a 20v DC supply that has a AC TTL control signal embedded into it.

I have a device that can read the TTL signals but i want to know what sort of circuit i need to place accross the 20V DC line to isolate the ttl signal.

I assume i need to build or ideally purchase some sort of coupling circuit.

I have basic electronic skills, EG i can follow a diagram to create a project on a bread board but i lack CALC skills for working out values of components.

The actual project that i am trying to do is to read the signals from my time clock to my central heating boiler as i want to take a more visual control over the boiler.

The vaillant ebus interface is the link I want to filter out the TTL information from


Can anyone point me to some links or give any advice on this subject

Thanks

Richard