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Querstion about RS485 bus design

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giovaniluigi

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I was reading about RS485 BUS design and I have some questions about the bus.

First I noted that I need to do a termination on the BUS trunk cable in order to finish the bus.

In a cascade configuration like:

[MASTER] -> [NODE 1] -> [NODE 2] -> [NODE 3] -> [TERM.]

The slave nodes are going to be added to the BUS and when you finish to add nodes you place a termination.

But, this is not a good thing to me. In my case I need to have a central node.
A PCB where all the nodes are going to be attached.
This is because I want to use a small and short cable for each node instead of the trunk cable passing through all the nodes.

My system will have the master node as a "Central node".
To do this I will let the trunk inside the master node PCB with a termination at the end of the PCB track.
In the middle of the PCB track I will place connectors to attach slave nodes.
Those nodes are not going to have terminations and they will use "long" cables.

rs485_trunk.jpg

My question: Is that scheme correct ?
Can I use RS485 this way ?
The trunk in my case is the shortest cable (actually a PCB track).
 

barry

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What you propose is not really the way 485 is intended to be configured. But that doesn't mean you CAN'T do it. How long is "long"? Just telling us it's "long" is meaningless. And what is your data rate?

Think about the REASON you need a termination: it's to minimize reflections which may corrupt your signals. You might want to actually try setting this up on the bench and looking at the signals with an oscilloscope. If your propagation delays are much less than your bit-width you might be able to get away without worrying too much about termination.
 

giovaniluigi

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barry,

In the jpg you can see the cable length. The trunk will be a PCB trace. The slaves have about 3 meters each.
The baudrate will be low (about 56700 bauds).
The purpose of the termination seems to be useless them in this PCB trace. Anyway I wil insert the bias resistor to the VCC and GND.

I pretend to connect each slave without any resistor.
I think that there is no problem on that and with just 3 meters of cable I'm not too much concerned.
I'm most interested on addressing capability of R485.

So can I just do the trunk using a PCB layer and then plug all the slaves on it whitout termination resistors ?
 

barry

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I think the length should be ok. The 3 resistors are not useless-they are a good idea because they pull the bus to a known state if no drivers are active.
 

HTA

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It might be necessary to terminate each receiving slave end with 120 ohm to reduce the reflection over the 3m line. If the DC load with 4 times 120 ohm it too much for the diver you could increase. The 120 ohm backplane termination should be removed(only the 680 to Vcc/Gnd need to be there). The best is to foresee all termination resistor and do actual measurements to see what is best, because the connector are another factor influencing line reflections.

Enjoy your design work!
 

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