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Smart Differential Receivers and termination

Started by k6ccc, March 05, 2021, 10:28:27 AM

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k6ccc

I am expecting a situation with a upcoming change in my year round landscaping lighting.  And that brought up a question about termination.  Let me describe my configuration.

I will have a F4v3 with the expansion board that will be driving a single 4 port smart differential receiver (set for smart mode A) that will be driving two pixel strings year round.  For Christmas, I will be connecting a second 4 port smart differential receiver (set for smart mode B) to the second RJ-45 on the first receiver.  I know that the last smart receiver needs to be set for termination.  That would mean that most of the year, the first (only) receiver should have the terminations turned on, and when the second receiver is added for Christmas, the terminations on receiver A would get turned off and the second receiver would have terminations turned on.

It would be FAR easier if I could instead build a termination plug that would be plugged into the second port of the first smart receiver most of the year, and when the second receiver is used for Christmas, the termination plug would be unplugged and the cable to the second receiver plugged in.  I don't see any reason that this would not work, and mechanically would be far easier than toggling the switches on the first receiver.  So my question is, what value of resistors?  My guess is the "standard" 120 ohm that is used for RS-485 networks.
Using LOR (mostly SuperStar) for all sequencing - using FPP only to drive P5 and P10 panels.
My show website:  http://newburghlights.org

Jim

pixelpuppy

Dave made some of the exact thing to send to people when a batch of boards got sent out with bad on-board termination.  You might check with him to see if he has and left over.
-Mark

JonB256

Electronically, it is a good practice to terminate RS-232 or RS-485. It is a "data integrity" concern, not a concern about damage to components.

I would suggest you try running the first one without termination. You will probably not have a problem with just the one. Running multiple Smart Receivers in a chain and not terminating the last one would probably cause a lot of data echoes. That isn't your described setup.
Long time Falcon, FPP and xLights user

k6ccc

I understand the purpose of terminating RS-485.  I do terminate all three of my LOR RS-485 networks.  Also understand that the longer the network, the faster the data sped, and the more devices on the network all increase the need for termination.  In my case, it will be about 125 feet from the F4v3 to the first receiver (that will be driving about 200 pixels on each of two strings) and maybe 15 feet to the second (300 pixels not evenly divided between four strings).

I will just try it without termination and see how it goes.  Easy to add if it's a problem.
Using LOR (mostly SuperStar) for all sequencing - using FPP only to drive P5 and P10 panels.
My show website:  http://newburghlights.org

Jim

k6ccc

#4
Quote from: pixelpuppy on March 05, 2021, 10:36:39 AMDave made some of the exact thing to send to people when a batch of boards got sent out with bad on-board termination.  You might check with him to see if he has and left over.

I remember hearing about that.  I have a hundred or so 1/8 watt 120 ohm resistors in stock and a large bag of RJ-45 connectors so I can build one in five minutes or less.  Not worth having Dave ship anything.
Using LOR (mostly SuperStar) for all sequencing - using FPP only to drive P5 and P10 panels.
My show website:  http://newburghlights.org

Jim

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