How to cancel Voltage Spikes in a digtal RS485 line?

[laser]

Hi all.
I´m working on a project in wich I use a very long line (500 meters or more) to transmit data in a diferential way (RS485 but with 25 volts).
Now, my problem is that I get some voltage spikes on the edges of the square pulses, that can go much higher or lower than the normal voltage (0 or 25).
This is problematic to me, since this can be catastrofic to my input circuit, that uses a comparator to read the diferential signals.
Is that a way to cancel these voltage spikes?
If yes, how?
For those of you interested, I have atached a screen capture of my oscilloscope, showing the problematic waveforms.
Thanks alot for your attention.

Francisco S.

 

[WA]

Before you input to comparators, I would run the pulse through a "clean-up" circuit. I would try a one-shot FF. This way, only the output from the one-shot (a cleaner pulse) can be compared.
I don't know if your comparing pulse width or just difference between 0 - 25V but, if you want same pulse width, you can determine external resistor and capacitor values from data sheet of FF.

Hope this helps
WA

To save bandwidth and postings I'm editing this post.
A FF is a Flip Flop circuit. Usually J-K, S-R D type ect...

Again hope this helps.
WA

 

[laser]

Sorry, but what do you mean with FF?

 

[flatulent]

A low pass filter will remove these.

It looks like the line is not terminated in the proper impedance (resistance). I would suggest that you first try lower resistances to see if you can remove the ringing without lowering the main signal level. Then try the low pass filter. One trick to lower the ringing without lowering the main pulse amplitude is to shunt the line with a resistor in series with a capacitor. You adjust the resistor for no ringing and adjust the capacitor for the maximum width of the full signal level. Another trick is to add a small resistor in series with the line at the driver end.

 

[laser]

I can try that, but one of my goals is to have the lowest load as possible on the line, since this system is intended to have about two hundred slave units or more, distributed along the line, which comunicate with a battery operated master unit.
So, I need to have the lowest current as possible on each slave unit (3 or 4 mA), in order to have a good time duration on the battery.
I will have to find a way to clean the signal, keeping in mind the current consumption limitations.

 

[flatulent]

If you are going to daisy chain the units, just put the termination on the farthest end of the line.

 

[risto80]

Hello
For killing the volttage spickes you must use ferite ring core,You can find him in olld pc power supply.Then put out the old coil and rewind with new.

 

[arcaphill]

The ring is because the line Q value ia too big!
Q=(√L/C)/Rs, Rs is the source resistor, L,C is the line parameter.
If Q=1, the Overshoot is 16%, Q=2, the Overshoot is 44%.
Make sure your PCB Q value less than 1, if possible, make Q=0.5

 

[marie65]

Add small value resistors at the output of A and B signal of RS485 (10 ohm), in series with the lines at each node. Add resistors termination at the ends of the long line to match the line impendance. Usually around 120 ohms but depends on the line type.

 

[eziggurat]

www.circuitcellar.com/library/ ccofeature/perrin0799/c79bppdf.pdf
pdfserv.maxim-ic.com/en/an/AN367.pdf
m.1asphost.com/ppluu/embedded/embed/rs485.pdf
www.embeddedsys.com/subpages/resources/ images/documents/microsys_art_RS485.pdf
www.dmx512-online.com/ddloads/RS485.pdf

Click on this link as well. This seem to be alot of good stuff for RS485.

**broken link removed**

 

[Humber555]

I´m working on a project in wich I use a very long line (500 meters or more) to transmit data
in a diferential way (RS485 but with 25 volts).

What do you mean with RS485 with 25 volts ? I don´t understand why do you use this
voltage level, it´s out of the expected value using standard RS485 tranceivers.
For a real RS485, those glitches aren´t a big problem because they canceal them.

Could you pls explain us what the reason to use 25 volts?

Regards,

Humber555