Unit & architecture

The unit

PROXON P 2H-L (Zimmermann Lüftungs- und Wärmesysteme, "P series" range): a central comfort ventilation unit with an integrated counter-flow heat exchanger and a frequency-modulated air-to-air heat pump. It is operated from a capacitive TFT panel that identifies itself as "BDE Comfort" in its system information.

The unit can heat, (optionally) cool and ventilate, and it has a controlled summer bypass (a standard component of the central unit, not an accessory) as well as PTC ceramic heating elements seated in the supply-air outlets for reheating on demand.

Two-board architecture

Two boards work together inside the unit:

Board	Core device	Role
Main board	NXP LPC1758 (Cortex-M3)	control, sensors, fans, heat pump, relays
PTC module 4×	STM32F1 + 4 power relays	switches the PTC heating elements; couples the two buses

The control panel talks to the PTC module at 9600 baud (over a Cat7 connection named X2). The PTC module in turn is connected to the main board through connector X7 — and that is where the actual control bus, HESP at 19200 baud, runs.

flowchart TB
    subgraph Unit
        direction LR
        X2["X2"] --- STM["PTC module<br/>STM32F1<br/>4× relays"]
        STM --- X7["X7"]
        X7 --- LPC["Main board<br/>LPC1758"]
    end
    BDE["TFT panel BDE Comfort"] -->|9600 8N1| X2
    LPC --- IO["Sensors · 2 fans ·<br/>HP circuit · solenoid valves ·<br/>bypass damper"]

The STM32 on the PTC module is therefore the active node between the 9600 baud panel side and the 19200 baud HESP side — there is no other path between the two domains.

The relevant bus level

Read and control access happen at the X7 / 19200 baud (HESP) level. The 9600 baud panel bus is not suitable for this (see Constraints & open points).

Connector X7

X7 is a 4-pole connector. Pinout (determined with a multimeter and a logic analyser):

Pin	Signal
X7-1	GND
X7-2	+12 V (VCC)
X7-3	Data B−
X7-4	Data A+

An off-the-shelf USB-RS485 adapter (in this investigation one with an SP485EEN transceiver, 19200 8N1) is connected as follows:

Adapter A+  →  X7-4
Adapter B-  →  X7-3
Adapter GND →  X7-1

A permanently fitted 120 Ω termination on the adapter did not affect the running communication in this investigation. The bus could be read passively and written to actively during normal operation without disturbing the regular communication (STM32 master ↔ LPC1758).

What is NOT on X7

• The T300 domestic hot water heat pump has its own controller and works entirely independently — it is not attached to the HESP bus.
• A device reboot is not triggered over X7 (no command frame was found).
• A ground heat exchanger is not fitted to this unit (fresh air comes through a core drilling); the optional "T before GHE" sensor is absent accordingly.