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About RS485 and 120 Ohm impedance

mulfycrowh , 04-18-2019, 09:03 AM

I have 5 boards using RS485 interface.
They are connected together from left to right.
I plan using Mil-Max connectors:
830-10-007-40-028000 and 801-43-007-40-002000.
They are 7 Pins connectors.
4 Pins are used for the RS485 interface, the other ones for GND.

The line has an impedance of 120 Ohm on each board.

I am worrying about the fact that the connectors will probably modify the impedance of the line.
Any suggestions ?

Thanks a lot !
Paul van Avesaath , 04-19-2019, 07:32 AM
dont worry too much about it.. RS485 is very robust.. how much distance is in between the connectors?
mulfycrowh , 04-19-2019, 07:34 AM
It's 1m40 for the distance
Paul van Avesaath , 04-19-2019, 07:53 AM
Then don't worries about it
robertferanec , 04-24-2019, 06:29 AM
I would not worry about connectors, but you may need to be careful what termination resistors you will fit (and maybe also what value ?). I have not worked much with RS485, but here you can find more info. Depends how your boards are connected, you may only need to keep termination ON on the first an last board(?) https://www.maximintegrated.com/en/a...dex.mvp/id/763
mulfycrowh , 04-24-2019, 06:30 AM
That's 120 Ohm
robertferanec , 04-24-2019, 06:31 AM
I mean, I am not sure if you can keep 120 on all the 5 boards. Check out the link.
mulfycrowh , 04-24-2019, 06:33 AM
No absolutely not: you have the first resistance on the first board, and the last resistance on the last board.
Paul van Avesaath , 04-24-2019, 06:39 AM
@mulfycrowh please just put a 120 ohm resistor at both sides (transmiter and receiver) on all PCB's its easier to remove the resistor than to add them this way you will be garenteed a working system.. and it will work don't worry too much..
robertferanec , 04-24-2019, 12:39 PM
just put a 120 ohm resistor at both sides (transmiter and receiver) on all PCB's its easier to remove the resistor than to add them
When I was using RS485, we used transceivers with build in 120R resistor which could be easily controlled through a digital pin (turn ON/OFF the termination). In different boards, we used a link/jumper to connect / disconnect termination resistor.

PS: Example of a chip with termination MAX13450E/MAX13451E (it is the first one what I googled when searched for "rs485 transceiver with termination"): https://datasheets.maximintegrated.c...-MAX13451E.pdf
mulfycrowh , 04-24-2019, 12:42 PM
I use SN65HVD33RHLR by Texas Instruments.
Paul van Avesaath , 04-24-2019, 12:52 PM
So when you read the datasheet you have the answer.. If you look at the typical application. Clearly it looks like point to point 120 omh parallel all in between not. (but take my advice and put one on all busses.. Because it's easier to take them off later than to add them)
mulfycrowh , 04-24-2019, 01:02 PM
Here is the schematics for first board
mulfycrowh , 04-24-2019, 01:05 PM
This the second board that could be the first one if Boesendorfer doesn't exist.
It could also be the third one ...
Paul van Avesaath , 04-26-2019, 04:11 AM
looks like you have done it correctly.. it will make sure you place them and if not needed then just remove them from the bom. might i give a suggestion?? try and give netnames a meaningfull notation. because calling it "A" and "ZZ" doenst give any notion on what you are routing. it does not matter if you have long netnames but it makes reading schematics (and reviewing pcb's) a lot faster
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