PROXON P series · The internal HESP bus¶
Scope¶
This documentation describes the internal RS485 communication bus of the Zimmermann PROXON P series (investigated on the P 2H-L model, counter-flow heat recovery with an air-to-air heat pump). The bus is designated HESP internally.
The aim is to describe the physical interface, the telegram format and the data points completely enough that an independent local read and control implementation can be built on top of them, on any platform. No manufacturer-provided route exists for this: according to the manufacturer's own statement, connection to building management systems is neither provided for nor retrofittable on the P series. The "BusBridge" known from forum posts applies exclusively to the FWT series with its integrated data bus (manufactured 2012–2018) and does not transfer to the P series. The interface described here is therefore the only known point of access. This documentation is deliberately implementation-neutral: it describes the interface, not any particular application.
System overview¶
The P series is operated from a central TFT control panel, which identifies itself as "BDE Comfort" in its system information. Internally, two boards communicate through connector X7 at 19200 baud, 8N1. On this bus an NXP LPC1758 controller provides all sensor readings, fan stages, operating modes and setpoints as typed data points.
flowchart LR
BDE["TFT panel<br/>(BDE Comfort)"] -->|9600 8N1<br/>Cat7| PTC["PTC module<br/>STM32<br/>(X2 ⟷ X7)"]
PTC -->|19200 8N1<br/>HESP| LPC["Main board<br/>LPC1758"]
LPC --- SENS["Sensors · fans ·<br/>heat pump · valves"]State of the investigation¶
| Aspect | State |
|---|---|
| Physical interface X7 (pinout, levels) | determined |
| Telegram format (header, types, lengths, data types) | determined |
| Checksum | determined as a CRC-16 (polynomial, init value, shift steps) |
| Reading data points over X7 | reproducible |
| Writing control data points over X7 | demonstrated, physical effect confirmed |
| Complete assignment of all temperature sensors | complete but for one sensor pair |
The points not yet conclusively settled are collected under Constraints & open points.
Applicability
All statements derive from measurements on one unit and were obtained without the manufacturer's involvement. Data point assignments and equipment features (for example cooling function, bypass, ground heat exchanger) may differ on other configurations. Working on the bus is done at your own risk and may affect warranty or guarantee. No proprietary manufacturer documentation is reproduced here.
Licence & trademarks¶
This documentation is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0): reuse and adaptation are permitted provided the source is credited. Short protocol and code examples may additionally be used without attribution.
This is an independent project with no connection to Zimmermann Lüftungs- und Wärmesysteme GmbH & Co. KG. "PROXON", "Zimmermann" and other names mentioned are trademarks of their respective owners and are used descriptively only.
Support¶
This documentation is free and grew out of many hours of measurement work. If it helps you, I would be glad of a coffee.