DC - supply over RS485

A project log for Digitize your analog Marklin HO model trains

DIY train decoder for Marklin analog HO trains

alexander deboualexander debou 09/13/2017 at 07:511 Comment

In this log I'd like to explain the concept to feed power over the data bus:

Their are a couple of configurations to combine the power supply and data on the same bus. It all depends oon seperating the power supply from the data.  The data is transmitted over a differential voltage across the two terminals of the RS485 driver (A - B) 

The picture above shows a possible configuration. A shielded cable is used to reduce noise when data is send over the cable. The seperation of the power supply and data is done by the capacitors C1,C2 and the inductors L4A, L4B. 

Simply said: 

The two capacitors will block the DC - voltage from the power supply while the inductors will act as a very high impedance towards high frequency signals ( RS485 data). It's important to choose the inductor value with the according baudrate or transmission speed of the data. 

The inductive reactance (XL = 2 * Pi * f * L) will increase with a higher inductance value, or higher frequency. It's important to lower the noise on the DC-power bus of the slave device as much as possible by using:

* The highest possible transmission speed 

* The highest possible inductance value

The transmission speed depends also on the lenght of the cable, while the inductance value depends on the type of inductor (core, without core) , and the number of windings. One problem is the physical size of the inductor. In my application of the concept everything has to fit inside a small place

(usable volume space: 20mm x 35mm x 9mm). 

A bigger value will result in a higer number of windings. A seconde note is the current through the inductor. Because the train will demand currents at 900mA, the inductors windings will have to withstand the current (also the current saturation value of the inductor is important). The high inductance value and the current rate will result in a relative big inductor. 

A downside of this configuration is signal noise of the power supply. All power supplies introduce some noise to the connected circuits. It's important that the power supply has some noise reduction circuits to minimalise the noise. This is important because the DC-voltage is supplied on both wires of the cable, and the capacitors to the RS485 driver only block DC-voltages noth noise (which are AC-components).

Other configurations cancel the signal noise of the power supply by connecting the positive terminal of the power supply to both wires of the cable and the negative terminal to the shielding braid of the cable. This implementation cannot be used because we connect the cable to Marklin tracks, which have only two seperated rails. 

Other signal noise may come from the motor used in these trains. It will be necessary to do some tests and choose good components:

* Determine the maximal length of the cable with the highest transmission speed and low data errors.

* Determine the ideal inductor ( important: type, physical size, current ratio). 

* A good DC - power supply with a low signal noise ratio or a good noise reduction filter. 

The design of the PCB only includes the capacitors that will block the DC-components on the cable. Because of the size of the inductors ( I'll be using axial inductors) they will not be placed on the PCB. 

This link explains different kinds of configurations that can be used to power devices over a RS485 bus:

(Picture used in this post came from this link and is edited a bit to be used in my application)


watercloudiot wrote 01/29/2020 at 13:13 point

where does the 120 ohm termination resistor go?

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