Simpler bus coupling design
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Author: pavkriz [ 11. Aug 2018 09:53 ]
Post subject: Simpler bus coupling design

I'm new to this project and I like it very much.

I would like to design a simpler (may be not necessary as much KNX compliant?) transceiver design.
Is the circuit explained more in detail somewhere? If not, I put some questions here:

1. What is the purpose of the 150mH inductor? Does it limit the DC current slew rate on the bus caused by the bus-powered device?

2. What is wrong with a simpler TX having only a MOSFET and resistor? See http://www.guiott.com/KNX/C-Hardware/KNXschematic.pdf and STMKNX https://www.st.com/resource/en/datasheet/stknx.pdf seems to have similar design (MOSFET inside and 68 ohm resistor outside).


Author: mff [ 23. Aug 2018 14:00 ]
Post subject: Re: Simpler bus coupling design

I can't give a good answer to your questions, perhaps oldcoolman or StefanT are the better contact persons for that.

But I found an interesting link some time ago, a transciever only with discrete components.
https://www.tapko.de/uploads/tx_neofile ... hys_01.pdf

Perhaps this could be a nice solution, but there's no schematic and I didn't really try to reverse engineer it from the pictures. ;)
Some hints only:
The used transistors according to marking code could be
https://assets.nexperia.com/documents/d ... BC856S.pdf
https://assets.nexperia.com/documents/d ... BC846S.pdf
The 5pin parts could be
https://assets.nexperia.com/documents/d ... SERIES.pdf

Too bad, the schematic shown here is very small.

Author: pavkriz [ 25. Aug 2018 20:47 ]
Post subject: Re: Simpler bus coupling design

I have two ideas to simplify TX and RX:

1) Use STCS05A https://www.st.com/resource/en/datasheet/stcs05a.pdf as 0,4A current-limiting TX switch. So the TX would be STCS05A (1€) + 4148 diode + 7V5 zener diode (dropping bus DC voltage by 7V, as specified for TP1-256 Transmitter).

2) Use MCU with integrated DAC and comparator (COMP). Positive input to COMP will be the BUS voltage divided by resistor voltage divider (to fit 3,3V COMP's rail-to-rail voltage). Negative input will be a reference voltage provided by internal DAC. COMP output will trigger capture channel of the timer in the same way as RX made of discrete components. DAC reference voltage may "follow" (calculated and controlled by software) the EIB_DC voltage measured by integrated ADC. So the RX would be actually made by only four resistors (the bus voltage divider and the DC voltage divider).

Any suggestions/concerns?

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