This repository is a work-in-progress implementation of microcontroller software (esp. Arduino Nano, 5V) to interact with a Junkers Cerastar ZWR/ZSR series gas furnace. These gas furnaces are antiquated, but some houses still have them installed. They lack any sort of sensible home automation interface.
The Cerastar mainboard exposes 5V I2C interface(s) on an on-board pad array and on two card-edge PCB connectors. One labeled "ST9 TA" accepts a Lumberg 2351 RAST-2.5 6-pin clip-on connector. The other labeled "ST7" has traces that connect directly to test pads above, labeled GND/5V/SDA/SCL. Whether ST9, ST7, and pads are the same I2C bus is unclear/untested.
Card edge connector "ST9 TA" officially accepts an obsolete Junkers-Bosch BM1 "bus module" (Junkers 8748300289, Junkers 8748300370). The BM1 bridges mainboard I2C interface over to an external CAN bus. The BM1 is based on a 1990'ies NXP P80C592 8-bit 80C51 MCU. The P80C592 MCU offers hardware CAN and UART (but no hardware I2C) and has a 2 x 256-byte internal RAM. One 256-byte of the page is apparently used as shared memory for communication between I2C devices (furnace mainboard, the BM1 itself) and official Junkers CAN bus devices (thermostats FR 100, TA 250, and others). This project emulates the BM1 256-byte memory. The CAN bus is (NB: eventually - currently absent) replaced by Wifi.
People on a mikrocontroller.net forum have traced I2C traffic between a Cerastar furnace and a real BM1. They were able to deduce most of the I2C user protocol.
For the whole background, see the thread https://www.mikrocontroller.net/topic/81265
For the source code used as a basis of this project, see the 05.03.2023 post by Joe L. (joe_l626) in that thread and his code, esp. junkers.cpp
- 1 x Lumberg 2351 RAST-2.5 6-pin connector (Digikey) plus pins, some cabling - or directly a Bosch 87144041510 cable from somewhere
- 5V microcontroller, Arduino Nano in case of this project, and som
- (future plan: 5V<=>3.3V I2C level shifter board, plus an Arduino Nano 33 IoT or maybe a Raspberry Pi Pico W wifi)
The RAST-2.5 6-pin connector cable assembly (or a Bosch 87144041510 cable) will be attached to "ST9 TA" (5V I2C bus) and to the microcontroller 5V, GND, SDA, SCL pins. No pull-up resistors needed.
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As described at https://www.mikrocontroller.net/topic/81265, ...
The code under ./src/cerastarI2C/ ought to compile as is under Arduino IDE. However, a patch to the Wire library is needed.
The Arduino Wire library has to be enhanced with minor code additions under pull request ArduinoCore-avr/pull/550, see also the corresponding patch or diff file. The stock Wire library in Arduino supports only predetermined transaction lengths. I created the patch to add Wire support for I2C EEPROM -like arbitrary-length memory reads and writes. Such transactions classically begin by providing a base memory address, and beginning from that base address, an "arbitrary" number of data bytes are sequentially read from target memory and clocked out from the target by the i2c controller (or clocked into the target by the i2c controller and written to target memory). The i2c controller can terminate the transaction at any point it is happy with the number (or the contents) of data bytes transferred so far.
The patch adds an OnRequestMore() handler for such open-ended transactions. An alternative method was, btw, suggested by ArduinoCore-avr developers. The existing OnRequest() handler could react not by Wire.write()'ing a single byte, but rather, proactively write 32 sequential data bytes (32 is the max. length of the hardware buffer). Wire by default discards any left-over buffer content when the transaction terminates "early". This works only if the i2c target never wants more than 32 bytes. This is probably true for the gas furnace i2c communcation.