Electronic – RS-485 differential signals won’t “cross”

RS-485 is differential, so looking at only one line doesn't tell you much. Such spikes on a scope trace can also be more a artifact of the scope than real, especially if the scope probe ground clip is not in a good spot. So the spikes shown on the plot may or may not be a real problem. You could try putting something like a transorb clamp accross the two lines to see if that changes anything.

What is more worrisome is the amount of noise shown, but again, we can't tell if that is common mode or a artifact of bad scope probe grounding.

It looks like your signal is only 0-3V. Is that what it's supposed to be? That could be right, but a lot of RS-485 implementations use 0-5V for both lines. If it's supposed to be 0-5V then something isn't right.

Added:

You now say the plot is supposedly showing the differntial signal because the scope probe ground clip was connected to one line and the tip to the other.

That doesn't add up. First, if you were really seeing the difference between the signals, then the peaks should be symmetric about ground. In other words, they should be -5 to +5 V, not 0-3 V. I suspect the scope is simply grounding one of the lines thru its wall plug ground, and then displaying the other using the wall plug ground as reference. That explains the single ended voltage level and the rather high amount of high frequency noise. Since the probe is not correctly grounded, the spikes might not really be there but rather a artifact of your flawed measurement setup.

You really know little about your signal at this point. At the very least, capture both lines independently on dual traces, and make sure the probe grounds are properly connected to the circuit ground near the transmitter chip. Until then, we really can't tell what is going on.

MAX9205, MAX9206 serializer and deserializer are a good choice if you want to "ship" several encoded bit-streams around. One chip serializes up to ten inputs and you can have a bunch of the deserializers in each of your speker systems. Because the serializer can use ten channels you have spare capacity to send a sync pulse down meaning no elaborate decoding of data is required. This picture shows 4 out of the ten channels being used - note that the inverse of the 4 channels are applied to inputs IN6 thru IN9. Serial clock (that will be a clock that is synchronous with the data) is also sent and so is WS (which I presume is like a word or frame sync for knowing when the MSBs are being shipped-out.

There are only two wires needed between the chips and I can vouch for them being good quality - there are also identical 2nd sources from National Semi and TI although I suspect TI aren't promoting the NS ones since the takeover.

How does it work? REFCLK is as a minimum 16MHz and each time the clock rising edge occurs, whatever is on the ten inputs is serialized up to a 160Mbpsec stream and sent over the twisted pair in a current loop. The 9206 reverses the process. You'll need to match xtals at both end but the can suffer a +/5% difference (from memory of tests I did when choosing them).

I've used these at clocks of 38.88MHz over a distance of 35m (very good twisted pair).

Conclusion about interfacing to DACs - Given that you can send serial data from ADCs and you can also simultaneously send the conversion start data I see no reason whay the 9206 has to interface with any microcontroller - the signals can be transported from the send end to suit most DACs like this: -