BACnet Transceiver Type - trouble finding information.

[derekd_paape_co]

Hello all,

I am always struggling with how to obtain the information I need to properly engineer a BACnet MS/TP trunk with multiple vendors.  Many of the support people I contact are not familiar with the term "Transceiver Type - Failsafe or Non-Failsafe, Isolated or Non-Isolated".  Does anyone else have the same issue? What tips can you share to help me ask the right questions to get the answers I need?

Thanks in advance.

[Shannon_Balch]

What information are you requiring?  Are you needing guidelines?  If so, are you needing information for a particular system?

[derekd_paape_co]

Hi Shannon,

Specifically I am looking to obtain the electrical properties of the RS-485 interface on certain 3rd party BACnet MS/TP devices.  I am trying to use Bob Schultz's guide from October 2016 to guide me to laying out the MS/TP networks.  My goal is to determine weather or not biasing is required, how many nodes I can have on each segment etc. I am just looking to hear what others have been doing trying to obtain this info.

[Jeff]

Derek,

Not sure if you've seen it yet, but I uploaded a 485 Transceiver list developed by one of our R&D engineers.  It certainly isn't comprehensive, but this could be a good way to track down specs based on transceiver used:

List of RS-485 Transceivers and Their Characteristics

--Jeff

[MikeGrinshpon]

Does anyone have a picoscope capture of a network that contains a non-failsafe transceiver?

[Jeff]

I have lots of them...they look incredibly similar to captures with all failsafe transceivers.  🙂  All captures including MNBs and SE7000s have non-failsafe devices in them.

If a transceiver is failsafe vs. non-failsafe, that typically just affects the way the transceiver interprets signals it receives...transceivers that are failsafe generally ignore an inversion of 485+ and 485- unless they cross by at least 250mV.  Transceivers that aren't failsafe can see much smaller crosses as a driven bit (hence the need for biasing to pull the + and - apart so they don't cross unless pulled to cross by an active transceiver).

If transceiver characteristics might affect the appearance of a waveform, I think the isolated / non-isolated distinction has a bigger affect.  Devices that are isolated don't use a local ground reference, so the waveforms don't seem as sharp.  Non-isolated devices, on the other hand, are often able to drive very crisp packets with very steady upper and lower voltage levels (from my experience).

Hope this helps!

[MikeGrinshpon]

Thanks Jeff;

I have been trying to call up manufacturers product support to get the transceiver info for devices we have integrated on the same bus as our b3's and so far have come up with a giant goose egg. If I can get the transceiver chip part number, I can easily find the datasheet on Google or look at your list and we are all good but so far there hasn't been a case where a device in the field has the chip just hangin out and easy to see. We obviously cannot go ripping apart devices that are operational.

How would you advise going about finding that info? Is there a magic keyword or sentence that we can use to pass tier 1 product support and go directly to their R&D department?

Regards,

Mike