RS485 daisy-chain

So we are running a RS485 network inside our cabinet. We have 4 rs485 devices inside. I've read that the network needs to be daisy chained (not star) topology. But nowhere does it explain WHY.

Why does RS485 need to be in daisy chain topology?

We've run RS485 in the past without terminating resistors. How necessary are they?

Please advise. Thanks.

Nearbyatom said:

So we are running a RS485 network inside our cabinet. We have 4 rs485 devices inside. I've read that the network needs to be daisy chained (not star) topology. But nowhere does it explain WHY.

Why does RS485 need to be in daisy chain topology?

We've run RS485 in the past without terminating resistors. How necessary are they?

Please advise. Thanks.

Click to expand...

You can have a star is the stubs are very short, have a read here...

https://www.ccontrols.com/pdf/ExtV1N1.pdf

http://www.ti.com/lit/an/slla272c/slla272c.pdf

Nearbyatom said:

Why does RS485 need to be in daisy chain topology?

Click to expand...

To avoid signal reflections that could corrupt data transmissions.

Nearbyatom said:

We've run RS485 in the past without terminating resistors. How necessary are they?

Click to expand...

As you've seen, many serial networks will function without the installation of terminating resistors, but some won't. It depends upon numerous factors such as wire length, etc.

The terminating resistors help to reduce reflections and electrical noise sensitivity. They aren't terribly expensive, nor are they very difficult to install. I always install the recommended terminating resistors, even if the network will function without them.

tragically1969 said:

You can have a star is the stubs are very short, have a read here...

https://www.ccontrols.com/pdf/ExtV1N1.pdf

http://www.ti.com/lit/an/slla272c/slla272c.pdf

Click to expand...

Thanks for the links. I just quickly glanced over it (I'll read into it later), but how short should the stubs be in order to use a star configuration? Wiring for a star is so much easier than daisy-chain. Our cabinet is 36x36. We will probably place the node in the middle part of the cabinet.

It's just plain bad practice to wire star topology with stubs.

Rs485

RS-485 is differential communications, staring or modified daisy chain cause reflections as do long runs hence the use of end of chain resistors. If you must us anything other then daisy chain they do main rs-485 hubs and repeaters that isolate and allow for spoke and hub(star).

nswu1 said:

RS-485 is differential communications, staring or modified daisy chain cause reflections as do long runs hence the use of end of chain resistors. If you must us anything other then daisy chain they do main rs-485 hubs and repeaters that isolate and allow for spoke and hub(star).

Click to expand...

+1

I was going to recommend that, although I don't really understand why wiring a daisy chain to 4 devices within a single panel is so darned difficult.

If you really want to know the "why", check out Jan Axelson's book; "Serial Port Complete". I can't recommend it highly enough.