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Testing RS485 bacnet


I need to do oscilloscope testing on BAS networks talking over RS485. Protocols like modus, BacNet, Johnson N2, and others. I need to know if your product line includes anything that can help with this.

The pertinent specs for my use I suppose is:

Voltage: +-5vdc all the time, with 24vac (typically 22v to 29vac in practice) on occasion if possible. 

Comm speed: 9600 bps, up to 115200 bps. I see 78600 and 38400 most frequently.

I will be most commonly be using an oscilloscope measuring differential between the + and - on the comm trunk, occasionally between + and a reference, and - and the same reference. The reference is usually the common leg of a 24vac transformer, but it can also be ground or a provided, dedicated reference wire. 

I need to use an Oscilloscope to find ground loops causing a voltage offset, noise from a VFD or lighting ballast, lack of termination, or improper voltage biasing, capacitance in the wire preventing the voltage from reaching the amplitude necessary to trigger a 0 or 1 value, shorts in the cable from rubbing or cutting that has voltage draining from a conductor to shield or between conductors. 

So first off, is the Analog Discovery 2 able to provide useful data? Is its oscilloscope able to provide readings on this type of network with good enough resolution that I can see the issues I am looking for?

Secondly, what is the best way to hook it up?

I was looking at the BNC connector board, as it provides the typical Oscilloscope probe connections. But then I noticed that it changes differential terminals into Single Ended ones and wondered if you may not have something better. For example, can I pull Oscilloscope readings from the Pmod RS485?


Or the Pmod-ad5?


My life is sitting on a bucket in a basement programming Air handlers and mechanicals... so the likelihood of shorting out this Analog Discovery 2 by touching a Single End ground clip to the wrong thing is plausible, and shorting out my controller's RS485 transceivers is likely. Crap happens in mechanical rooms. I would much rather use differential connections. If I could add the to Analog Discovery 2 to the trunk using the Pmod-RS485 and scope it - that would be beyond perfect. 

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2 hours ago, jpeyron said:

Hi @Numbawunfela,

I moved your thread to a section where more AD2/waveforms experienced engineers look.

thank you,


Thank you sir!

I did a little looking around and saw that the bacnet testing labs use the picoscope 2204 to determine conformity to their spec.


The specs are similar to the Analog   Discovery 2 , a little worse. So I am pretty confident the oscilloscope feature is up to the task. I also know an Oscilloscope can detect all the stuff I mentioned in the last post. 

So now it is a matter of hooking it up. Since digilent makes the AD2 and the Pmod RS485, I figured they would go together. Is that the case? 

Also, the specs have the AD2 with Oscope inputs rated at 25v. If I come across 28vac to 30vac, with the AD2 go poof? Or with it just not like me for a while, but get over it? 

I think I need to know more about the interperability of the hardware, and how it behaves in the wild.

Edited by Numbawunfela
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Well, I kinda figured the presales forums got more traffic....

The details on the AD2 say the inputs are rated for 25v, but are protected to 50v. So it will not go poof at 30v. 

It looks like I can hook up the Pmod RS485 however I want, as long as I tell the Waveforms software where to look for the pertinent inputs. It also has the benefits of overvoltage protection that is pretty high, protecting the AD2.

It sure would be swell to get a Digilent person to be a little more Diligent and give a thumbs up or something. I have constructed and tagged the thread so another person doing Building Automation will find the thread easily. 

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I am looking at the protocol analyzer. I have been through the waveforms manual and the SDK documentation, but I cannot see the answer anywhere. Does Wavforms have the ability to add 'Link File' like Picoscope does?


"Link File helps to speed analysis by cross referencing hexadecimal field values into human readable form. So, for example, instead of displaying "Address: 7E" in the Table View, the corresponding text “Set Motor Speed” will be shown instead, or whatever is appropriate. The Link File template with all field headings can be created directly from the serial table toolbar, and edited manually as a spreadsheet to apply the cross reference values."

I see that there is a custom protocol editor, but that does not appear to be the same thing....

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