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Old July 7th, 2012   #4
ericthegeek
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Quote:
Originally Posted by ELMVideo View Post
As for question 1 above, I'm asking in a general sense. The E1.20 document shows a reference circuit with Rt2 (120 ohm) and the biasing network, I just want to make sure I'm understanding where the two belong. The biasing network should be (for a 1 in 2+ out circuit) on the 2+ outputs (simulating controllers) and the (switchable) termination on the one input of this type of circuit?
Yes, you are correct.

The formal RDM terminology is "Command Port" and "Responder Port". A 1-in, 2-out RDM splitter would have 2 Command Ports and 1 Responder Port. The two command ports would have the line biasing network described in Figure 2-1 with R1, Rt1, and R2 built into the splitter for each port.

The 1 responder port may have the switchable 120 ohm termination.

See the attached diagram for details.

Quote:
Originally Posted by ELMVideo View Post
As for the 485 IC's (75176) the 'R' data out of theses IC's need to combine to 'simulate' a collision. Just to verify,
You can combine them to simulate a collision internally, but you don't have to.

In RDM, collisions will only occur during the discovery response period. On a single 485 segment, the effects of a collision are primarily analog with multiple 485 line drivers driving both high and low.

But, when you have a splitter, there can can be one or more responders on each 485 segment connected to the splitter. That means each segment will have its own analog effects. So which should you pass back to the responder port? You can AND all of the downstream segments together and drive that on to the responder port. Or, you can pick any segment that has activity (Typically the first falling edge on any downstream segment) and pass that segment back to the upstream port. Either is fine. The console should interpret *any* activity as a positive response and continue further down the discovery tree.

Technically picking one port rather than ANDing them all together can result in slightly faster discovery because it will cut down on the number of collisions. The first port to see a response may have a clean DUB response that the controller can decode and then mute the device directly, and other ports won't have a chance to interfere. But it practice the difference is minimal. (In the attached diagram, if responders #2, #3, and #4 all tried to respond, and the top port on the splitter triggered first, the controller would see a clean DUB response from #2 since #3 and #4 wouldn't have a chance to collide)

One thing to watch out for: You need to make sure that you account for other responders being present on the same segment as you splitter's upstream port. You can't just blindly turn on the Transmit Enable for the responder port during the discovery response period because it can mask valid responses from other devices. You have to wait until you see an edge on one of the downstream ports and then turn on Transmit Enable.

Using the diagram, if only #1 is responding to a DUB request, the splitter should not turn on its transmit enable.


Quote:
Originally Posted by ELMVideo View Post
Thanks again,
Glad we could help.
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