I'll give this a try.
The C-80 Systems speaks "Seatalk" (NMEA 0183 Raymarine Version) Seatalk uses a small rectangular plug.
The Raymarine Forum has posted a support thread that will provide a couple of manuals to help you understand the system. Here is the link;
RayMarine C80
Your new EV-100 AP talks SeatalkNG... (a foriegn language for the Seatalk speaking systems.) You will need a translation cord that plugs into the P70 AP Control unit using the plug designed for SeatalkNG and at the other end of the cord is a plug that will plug into one of your "Seatalk" displays.
See the system image from the EV0100 AP manual here:
It shows that item number 6 is an adaptor cable. It is plugged into item 1 (SeatalkNG P70 AP display) and item 2 (Seatalk ST60+ Depth display). Because the ST60+ is connected to the Seatalk net through the other instruments, you can get the AP signals sent to your C-80 Display if it is connected to the Seatalk instruments.
That is the theory. You will need to test this out to see if it will work for you. Working with year 2006 designed functioning systems and then trying to connect them to 2016 speaking systems is sometimes shear luck.
Like trying to get a PC running windows 3.1 to work with an iPhone 8. In theory it should be able to happen, but in practical terms you might not want to try.
Your AP working with the R-70 will function without talking to the C-80. You can set the course to a magnetic bearing and the will go in that direction until
- Give it a new bearing,
- Tell it to stop - Setting the P-70 to Standby
- Or you run aground.
Hope this helps.