Electronics > Beginners

RS485 Ground isolation

(1/3) > >>

hsn93:
hello,


I've Rs485 <=> usb which i connect to the PC.

and power supply that is powering my board.

Im running my signal (A+B) + power (24V + GND) through this cable (< 5meters)

http://www.cnctech.us/pdfs/2000-2422-1-BL-0025-TS.pdf


the shield is not connected to anything from both sides  :)


- the lab power supply is isolated than the PC obviously.


if you notice the RS485 has Gnd wire

if i common ground the lab-psu and the (usb-485) .. everything works good
however if i keep them isolated the signal is unrelaiable and noisy.


i dont use (isolated RS485) but still
according to my understanding RS485 measures the potential between A and B (ie Return path is A or B)
so why its affecting the communication here?

mariush:
You're supposed to use twisted pair for data. That cable doesn't seem to be twisted pair.

See https://www.lammertbies.nl/comm/info/RS-485.html

capt bullshot:
Non-isolated RS485 needs the common GND to work reliably. Even with isolated transceivers it's recommended to have a common GND for the isolated side. The receiver side does indeed receive the difference voltage between A and B, but isn't capable to deal with larger common mode swings that may easily occur if their GND isn't connected.

hsn93:


--- Quote ---Non-isolated RS485 needs the common GND to work reliably. Even with isolated transceivers it's recommended to have a common GND for the isolated side. The receiver side does indeed receive the difference voltage between A and B, but isn't capable to deal with larger common mode swings that may easily occur if their GND isn't connected.
--- End quote ---
im thinking,

so from my understanding the return path is in A,B but still the IC chips work with some common ground (basically because of "low common mode noise rejection" ?

i just noticed that usb ground has around (100VAC @ 50Hz) with the lab power supply ground , + , earth.

this means the PSU of my PC is not isolated ?
or it could be isolated but some capacitor is there for some reason .. which reason??



--- Quote ---You're supposed to use twisted pair for data. That cable doesn't seem to be twisted pair.
--- End quote ---
yeah i cant find on the datasheet i have to check physically because i remember that i picked it up specifically for RS485 with the impedance 120 Ohm

Paul Moir:

--- Quote from: hsn93 on October 15, 2019, 03:01:57 pm ---
so from my understanding the return path is in A,B but still the IC chips work with some common ground (basically because of "low common mode noise rejection" ?

i just noticed that usb ground has around (100VAC @ 50Hz) with the lab power supply ground , + , earth.

this means the PSU of my PC is not isolated ?
or it could be isolated but some capacitor is there for some reason .. which reason??


--- End quote ---

Inside the laptop brick is a capacitor across the transformer which is used to reduce EMI.  It also leaks some AC into your laptop's ground. 
Your understanding's correct.  You don't say how fast you're running but twisted pair isn't going to matter over that short distance.  However do make sure you have your termination resistors in place; you'll have to guess at the impedance of the cable but if you are a little off it won't matter much.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod