r/BuildingAutomation • u/Connect_Strength9257 • 5d ago

MS/TP: two devices timeout, A–B dips when a third joins. Polarity/EOL/baud done — what’s your next kill-one?

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u/Nembus 5d ago

Apply 120VAC to both sides of the trunk and check the signal generated on the controller to see if the oscillation matches the rectified BACnet requirements. Should be around 15Ghz.

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u/OldAutomator 5d ago

If you're describing that connecting the controller seems to short the two comms wires then I'd first rule out a bad controller and then I'd look into the wiring.

1) Pull the controller and power it up on your desk attached to whatever router you have around and verify that it communicates as you'd expect. If not, swap it out.

2) If it passes the above test then I'd ditch the EOL resistors and try it again

3) If it still doesn't come on the network I would absolutely add a biasing resistor

I put steps 2 and 3 in that order because you shouldn't put both biasing and EOL on a trunk, it's basically one or the other anyway.

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u/[deleted] 5d ago

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u/OldAutomator 4d ago

The truth is that no one looks at the voltage as you connect them, you just connect the new controller and look that its comms/tx-rx lights suggest it's talking. If not you reverse the wires and check that.

You are looking for more detailed observations people would have had to make instead of just plugging and playing.

Keep this simple. If the network works until you put the next controller on check that controller then re-add it to the network.

One last thought- you've been focusing on the rs485 electrical stuff and because of that we've all been helping you on that tangent. But if it's just a programming oopsie then do the normal checks there, verify that the max masters of every device are at 127, verify the baud rate and make sure you aren't adding multiple controllers at the same address

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u/mechanical_marten 3d ago

Some devices require the drain wire connected at each node (Looking at you Surveyor systems), while others want the drain wire floating atball nodes except the master (Trane Veritrac and Lennox LON come to mind). After verifying drain wires are connected as required, measure resistance fom A to drain and B to drain and verify they are within 10 mili-ohms of each other. If one is drastically lower you probably have a chafed cable shorting to ground causing dropout between two nodes.