Open collector differential signalling, choosing the better option

[Infraviolet]

I'm looking at a situation where I want to have multiple sending devices able to pass a differential signal through a long distance in a potentially noisy environment to a receiver. As there are multiple senders I want to ensure electrically that there is no possibility of over-current damage from one sender taking a line low and the other taking it high (via push-pull signalling) at the same time, so I'm going to be using some form of open collector. I have got "protocol" stuff at higher levels to deal with making sure that actual data sent by different devices doesn't clash with one-another, so this forum thread is just about the electrical level.

So, for open collector differential signalling I've got two choices. I can have, for a differential pair, both as NPN collectors operating in opposition, or I can have one as an NPN open collector and the other as a PNP open collector. As far as the comparator at the receiving end is concerned there's no real dfference, but in terms of the type of output used to do the sending, and whether each line of the pair has a pulldown to Gnd or a pullup to the positive voltage rail, there is a difference.

Which then, see the diagram attached, is the better choice for noise immunity? Or if they are equivalent in that regard, which is "better" by other measures, for example the dual NPN type would always be consuming power supply current through one or the other pullup at any given time, while the NPN+PNP one would consume twice as much current when the PNP line was driven high (and the NPN driven low) and no current when in the other logical state (PNP released to be pulled low and NPN released to be pulled high).

Which choice is "better", and if noise immunity is identical then for what reason is one of them a better option.

Thanks

[Infraviolet]

Any thoughts, I can't seem to find anything when searching regarding differential signalling with open-collectors. All the results I find seem to just be about push-pull differential versus open-collector with a single wire. Thanks

[langwadt]

CAN bus ...

[Infraviolet]

Thanks for suggesting looking at CAN bus instead of searching for open collector and differential signalling specifically, CAN looks to use the mixed "nppn" and "pnp" method. I guess that's the "better" choice then, if CAN chooses to use it, is there a reason why for this over the all-npn-method though? Thanks

[langwadt]

Thanks for suggesting looking at CAN bus instead of searching for open collector and differential signalling specifically, CAN looks to use the mixed "nppn" and "pnp" method. I guess that's the "better" choice then, if CAN chooses to use it, is there a reason why for this over the all-npn-method though? Thanks

mixed one state both + and - is driven other is pull up, with all npn both states have one or the other side only pull up

the first CAN transceivers where basically RS485 transceivers using the enable pin for data and the data pin fixed low

[Kleinstein]

The case with both NPN would have current flowing through ground, not just the differential pair. The PNP NPN combination avoids the ground current. As a downside one still needs to make sure the lines are not swapped anywhere.

Ideally a long line should be terminated and the swiching transistors would be pretty low impedance. One may want some extra current limit, possibly just a current signal, so that the termination is passive on both ends and not from the drivers.

[selcuk]

Is this circuit for experimental or educational purposes? If it is for an actual device, I recommend you to use a CAN transceiver. They have measures for bus collision, high common voltages, ESD, fast rise times etc. A reliable solution with discrete components will need a good amount of PCB space.