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scs_bridge hardware

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krahabb edited this page Aug 24, 2021 · 1 revision

Interfacing to SCS bus is done through a basic adaptation circuit.

The issue here is the bus voltage around 27 V which might be dangerous for the ESP. An optocoupled solution would definitely be better but this is my implementation based on what I had available. Also, the output transistor (Q2) has to draw a bit of a current so it might heat a bit (together with resistor R4). You could add a DC-DC step down to power the 3.3 V ESP rail so to self power the circuit (a good stabilized voltage source, around 27V, could be C1 which already provides some buffering).

The bus thresholds are unknown to me and also the exact waveform timings so this implementation could not work realiably everywhere. GuidoPic did some testing and came out with a different setup than mine (expecially on R4). In the beginning I also had some troubles thinking the specifications where too tight. In the end this doesn't look so anymore and this implementation worked well and doesn't feel 'tight' anymore

Schematic

Components

values in brackets are from my implementation but those could be easily substituted

  • D1: general purpose rectifier (1N4007 or so)
  • Q1: general purpose PNP BJT (MPS2907A)
  • Q2: medium power NPN BJT (2N3904 this is a bit underpowered for the task) Q1 and Q2 have to withstand at least 30V Vce and Q2 is dissipating some heat (Ic around 100 mA when conducting). Some heat also goes on R4 (I've cooked them a bit in my trials) C2 and C3 provide some smoothing but will also distorce the waveforms. These values work good in order to prevent spikes (expecially C3 avoids a lot of bus noise when Q2 switches). Try to keep their values as set, together with their paired resistors, in order to not exceed bus timings tolerance. If you fall in trouble you'd better ask for an 'electronic friend' since troubleshooting should be done with oscilloscopes and the likes. But if you're down this way to interface your SCS system you probably know what you are doing ;)

Warning

the SCS bus has polarity and my circuit will dislike a reversed connection: watch out!

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