TDC 2000/3000 used a proprietary protocol created by Honeywell. You can find it fully described here: https://www.honeywellprocess.com/library/marketing/product-manuals/HG03-400.pdf.
Before asking questions like this, please try to find your own answers the way I do - Google it! To find the above document I searched for TDC 3000 Data Hiway. The first item in the Google list was the above PDF.
Please clarify your application.
Internally I have no idea. Another post provided some info. The HPM I/O has it's own interface. TDC has a mux or bridge that talks Modbus for PLC's.
If you google it you will find literature. Sign up with Honeywell's Automation web site, they have all their manuals online.