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Old October 12th, 2017, 12:38 PM   #1
nick88
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RS485 Communication Issue

I have a Redlion G315 HMI dsplay with a RS-232 serial communication link using an ABB TotalFlow Master driver. The RS-232 serial line is hooked to a B&B Smartworx RS-232/RS-485 isolated converter. 2-wire RS-485 link wired as a daisy chain is used to talk to three different nodes in the field. Total distance of communication link is probably around 600 ft or so. Im using a single pair shielded 16awg.

Each node is a Totalflow RTU. I do have the termination resistor jumper set on the last RTU. I am using the Redlion to poll app-array-registers out of the RTU's and display on the HMI.

I am able to poll the first RTU with no issue, but if I try polling the polling the second RTU, I loose communications. I do not receive any data from RTU's one or two.

What are some troubleshooting methods I can try to see whats going on?

Nick
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Old October 12th, 2017, 01:46 PM   #2
KuulKuum
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Quote:
Originally Posted by nick88 View Post
I have a Redlion G315 HMI dsplay with a RS-232 serial communication link using an ABB TotalFlow Master driver. The RS-232 serial line is hooked to a B&B Smartworx RS-232/RS-485 isolated converter. 2-wire RS-485 link wired as a daisy chain is used to talk to three different nodes in the field. Total distance of communication link is probably around 600 ft or so. Im using a single pair shielded 16awg.

Each node is a Totalflow RTU. I do have the termination resistor jumper set on the last RTU. I am using the Redlion to poll app-array-registers out of the RTU's and display on the HMI.

I am able to poll the first RTU with no issue, but if I try polling the polling the second RTU, I loose communications. I do not receive any data from RTU's one or two.

What are some troubleshooting methods I can try to see whats going on?

Nick
Nick,
Is this a new installation or an existing one that used to work?
You also stated single pair shielded 16awg (is this "twisted" pair?) also is the signal grounded to a common ground?
Another idea is to provide terminating resistors at each extreme end of your communication link.
Check you communication settings (half duplex, baud rate, parity, etc.)
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Old October 12th, 2017, 06:09 PM   #3
danw
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RS-485 does not like spurs. A spur is cabling which the tees off the main line and to connect to a node on multidrop network. Reflections will create comm issues. The main line network should connect to the node directly and then go onto the next node from the same node connection terminals.

Be aware that a lot of devices do not register or acknowledge changes in serial settings (baud rate, parity) unless the device is power cycled.

The cable should be twisted pair.

Comm cables should not run parallel to or in the same conduit/tray as power wiring.
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Old October 19th, 2017, 06:04 AM   #4
nick88
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RS-485 does not like spurs. A spur is cabling which the tees off the main line and to connect to a node on multidrop network. Reflections will create comm issues. The main line network should connect to the node directly and then go onto the next node from the same node connection terminals.
This network is wired in a daisy chain pattern, so there are no tees branching from main trunk line.

Quote:
Is this a new installation or an existing one that used to work?
This is an installation that used to work. Used the RS485 port within Redlion, until this became damaged. I'm guessing noise or transient voltages from circuit damaged this port. That is why I decided to install isolator.

On my old installation, I could only get the 2nd and 3rd RTU's transmit, if I tried polling the 1st RTU, comms would drop out

The cable is laid in DC conduits, no power wiring.

I believe the pair is twisted.

Unfortunately, I do not have a spair pair to use for a ground, so I'm using the drain shield as the gnd. I know this is against common practice, but it seems the only way I can get the circuit to communicate.
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Old October 19th, 2017, 09:29 AM   #5
danw
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I suspect you are facing a common mode voltage difference on the signal grounds.

An analogy is a single ended analog input card with, say, 8 inputs, where all the signal grounds are common and are essentially the chassis ground. Someone connects three analog inputs and they all work fine. A fourth input is added and as soon as its wires connect, all the inputs are driven offscale.

The problem is common mode voltage between the difference source points. The practical solution is isolation.

I suspect that the RS-485 signal ground has different signal ground potentials at different locations and that is creating the problem that the signal amplitudes fall outside the detectable range.

If it were me, I'd do the following.

I'd test each drive independently to make sure that the RS-485 port is still functional, because, after all, the master port failed. No sense chasing solutions only to discover that any given drive's comm port is also dysfunctional.

You know that you have a working drive #1, master and 232/485 converter.

Test drive #2 as the only drive on the network:
Disconnect Drive #1, reconnect the signal wires. Leave drive #2 connected, disconnect drive #3. Now drive #2 is the only connected device on the network. Does drive #2 communicate with the master?

Test drive #3 as the only drive on the network:
Do the same with Drive #3 by disconnnecting drive #2, reconnecting the signal wires and re-connecting drive #3. Does drive #3 communicate with the master?

Go buy at least one stand-alone RS-485 isolator.

Assuming drive #2 is still the only drive connected to the master (it doesn't matter which drive you test with, but do the test with only one drive) install the new isolator and confirm that you have connected it so it works. This eliminates the issues with drive line labeling and power. It proves the isolator works and how to connect it.

Then re-connect drive 1 directly to the network signal cabling and and install the RS-485 isolator between it and one of the other drives, either drive #2 or #3. Leave the remaining drive unconnected for the teste.

Odds are, you'll get communication with both.

You can try adding the 3rd drive, but it might or might not work, depending on how the common mode is created. You might need a 2nd isolator to isolate the 3rd drive.
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