RS-485 Unterminated Bus and bias resistors

After looking at several application notes (by National, TI, B&B Electronic...) I can't understand if I can bias the two-wire bus without inserting a 120 ohm resistor between A and B cables. Reading B&B Electronics

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it seems to be possible (pages 18 and 19). Reading National
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not (pages 8 and 9).

My design doesn't need terminators, as total cable length will be less than 4 meters, but I have to guarantee fail-safe conditions in bus idle state.

Thanks

Reply to
djordj
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you can.

If you don't rely on the resistors in the transceivers, use one resistor to 0V, one resistor to Vbias (e.g.5V).

Depending on your speed requirements, you might want to add one resistor between the signal lines to speed up an eventual transition from "space" to "idle" (if you can't rule out this possibility).

Oliver

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Oliver Betz, Munich
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Reply to
Oliver Betz

This exactly is the solution I'd like to use, as we don't want to constrain the system builder to insert the final terminator (this would be a dangerous option left to the final user........ sadly...). So, only pull-up and pull-down: nothing else.

Well, because of underlying hardware (CPUs) the bus will run at a relative low speed (115200 bps), so I don't think that we'll have the need to speed-up "idle"-"active"-"idle". Another question: this optional resistor doesn't need to be balanced by a terminator?

Thanks!

Reply to
djordj

And where do you want to place these resistors? At every node, or do you have a single 'master' node where you can put them?

There are also 'fail safe' RS485 tranceivers, but the fail safe does rely on the differential signal being close to '0' -> terminator.

And in which electrically clean environment will this be used?

In noisy environments, I would really add terminators. Educate your customers/system builders. I've seen RS485 fail with 'just' 115k2 and 'only' a few meters of cable. Adding terminators can really make a difference. And make sure you use nice, twisted, cable.

If you let others wire up the systems, document how it should be done. Type of cable, lengths, stub length, terminator placement, etc.

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Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

It was all so different before everything changed.
Reply to
Stef

We have a master board, where bias resistors will be placed.

It's a closed body machine, with no motors but some static relays, EVs and heaters.

I usually follow those rules: luckily boards will be cabled using predetermined connectors, so the machine builder has only to follow connectors color scheme in order to get the right cabling. There will be non "visible" stubs, as RS485 wires will be routed to transceivers really close to the connectors, via PCB tracks.

The only problem is the terminator placement: the first can be factory placed on master board, but not the second, as the machine can be built of two to four slaves devices. We have to hope that the machine builder will not forget to close that holy jumper... :)

Reply to
djordj

I think at 4M almost any configuration will work....I ran 100M + at 115K no problem, but yes, if you need failsake use the pull up and down resistors.

Reply to
TTman

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