Wiring practices

Wiring practices

Postby krokodil » Mon Feb 02, 2015 5:02 pm

Hi
let me share you some new experiences with modern processor driven circuits on model railroad. (Switch machines, automated train controls etc.)
On almost all new devices the inputs are scanned on relatively high impedances and very fast ( too fast for my taste). For that reason wires over 10-18 inch (25-40 cm) can be already an issue. For example switches/pushbuttons on the control panel, or remote track sensors ( does not really matters what kind - let us call them activators) on long wires will be ignored by the electronics.
Therefore place them as close as possible to the activators, and run longer wires rather to the controlled devices like switch machines, track segments, lights etc.
It reduces the possibility of heavy headaches - why the device works perfect on the workbench and does not on the layout.

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Re: Wiring practices

Postby ConducTTor » Tue Feb 03, 2015 6:41 pm

Interesting scenario. Good to keep in mind for the future. Thanks for the heads up!
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Re: Wiring practices

Postby Tom Dempsey » Tue Feb 03, 2015 7:19 pm

That makes no sense to me electrically, are you sure that you are not getting an interference issue. The currents are so small that if you run the wires next to each other, signals can get cancelled out. Try twisting the wires or better yet putting about an inch or so between them in the runs.
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Re: Wiring practices

Postby krokodil » Tue Feb 03, 2015 8:23 pm

I spent almost 2 days investigating. It was really the last, to rebuild my nice small control panel, but there was no other way. I tried different cables, twisted, even a shielded twist pair, no influence. Cables longer than 40 cms created uncertainity in operation. My control panel has an about 100cm long flexible 25 way cable. I made it from individual wires and i replaced those 8 control wires with different other options). The switches on the control end pull down the inputs to ground by 330ohms, and this should be detected by the input circuitry of the processor. The ground was very strong ( several wires in parallel or the shielding of the twisted pair). It was interresting to see that one input alone worked fine, when I added the next one I saw already some degradation, i had to push longer the button to switch. At four inputs the individual pushbuttons became very strange sometimes they were detected sometimes not. In my case the pushbutton toggles the input, ie. At first hit it turns on the output on the second one it turnes off. When all 8 inouts were wired usually I could turn off the outputs, but I could not switch them on.
I think, (I did not measured yet by scope), the procesor switches on and off the input pins ( sometimes they are inputs, sometimes outputs) and those pulses in the wire are radiated and create noises. This processor is not my design, I talked to manufacturer, he recommended to pull the wires in about 1 inch distance, unfortunately this is inpracticable when the control panel is connected by a muliway patchcable to the main layout.
I am almost also done with my own processor design, I will check this effect on this processor as well.
Anyway the short cable solved my problem, this means from the portable control panel I have now only output signals.
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Re: Wiring practices

Postby harmi » Thu May 18, 2017 9:02 am

All inputs should be pulled-up to +5V through relatively large resistors (compared to mentioned 330ohms) and sufficiently small against impedance of inputs (MOhm), so e.g. 10k-33k. This will reduce sensitivity of inputs. And if not, add capacitor 100nF between input pin and ground. Although I think that debounce function was supposed to be implemented in processor programm (e.g. LOW or HIGH level lasts at least for 250milliseconds to be taken into account).
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Re: Wiring practices

Postby krokodil » Thu May 18, 2017 10:02 am

Yes, all those measures were done ( debouncing, pull-up resistors) even the wires are mostly twisted. (Not always is possible). Still the input side is definitively more sensitive for overall noise ( we have enough on model railroads) than the output side.
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