Installing Nest on Viconics P117 Board
It looks like the P117 + E117 was an intelligent system. Sort of like a NEST, but in 2 parts. This is different from the typical (in the US at least) thermostat where the thermostat - whether an old Honeywell dial, a typical digital setback thermostat, or a newfangled NEST - communicates to the HVAC equipment using a fairly standard set of wires. From the NEST manual:


Heating: one, two, and three stages (W1, W2, W3)
Cooling: one and two stages (Y1, Y2)
Heat pump: with auxiliary and emergency heat (O/B, AUX, E)
Fan (G)
Power (C, RH, RC)
Humidifier or dehumidifier (HUM, DEHUM)


You can actually see some of these wires in the P117/E117 diagram. For example, the upper right shows connections to C/X (the /X doesn't match NEST), R, G, W2; the right shows C, R, O/B, Y, W.

The problem is you can't just rip out the P117 and E117 and put in the NEST because you only have a 3-wire connection (which is all two computers need to talk to each other - equivalent to an old serial port send/receive/ground). You will likely need to find a way to run a new cable with more wires from the actual HVAC equipment directly to the NEST and then you can remove BOTH the P117 and E117.

Based on your comment that you already have a cable with the standard colors, you need to:


Find and label all connections from your HVAC equipment to the P117.
Replace all the connections from the P117 with the appropriate thermostat cable wires.
Connect the NEST to the matching wires.


One critical piece is power. If I understand the diagram correctly, power is supplied on C/X and R in the upper right. The NEST needs power and that is listed as C, RH, RC. So C/X --> C and probably R to your choice of RH or RC. But I can't say for sure. The good news is that it looks like the C/X - R is 24V AC, which is standard for the US.