BAMTV 006: BACnet MS/TP Overview

[@mabaigkhusro]

Dear Mr. Phil, thanks for posting such an informative Video.

[@2540mahen]

Excellent expalation in a very simple way.Thank you

[@SmartBuildingsAcademy]

Thanks I'm glad it helped!

[@Odiee47]

I built an implementation of BACnet Protocol Standard for my company (closed source) that includes MSTP comm. layer. So I basically already know everything that was explained in this video. But if you told me to to put all that knowledge in a precise and concise presentation such as this, I would tell to to F*** off. Watching this video, there were multiple times that I found myself thinking: "Oh yeah! I forgot about that." The video even goes into explaining baud speed, cable length and packet loss. Fantastic job.

[@SmartBuildingsAcademy]

Thanks, I believe that folks would find MS/TP a lot easier if folks in our industry had a deeper understanding of what goes on behind the scenes with the protocol.

[@Odiee47]

@@SmartBuildingsAcademy Not to mention that The Standard is pretty technical about it, so all the theory so succinctly presented in this video would be a godsend 10 years ago when I made it. "Here all you should know before trying to make MSTP comm. layer." I would've cried. You basically had 100 pages of MSTP technical specs and Steve Karg's implementation in C. So good luck.

[@kalpase2006]

I wanted to just check something but the video was so instructive that I watched the whole video, the only thing I did not understand was the difference between segment and trunk as you use them interchangeably, and while the limitation for the segment was 4000ft for trunk you mentioned 15000 ft?

[@SmartBuildingsAcademy]

A trunk can consist of 3 segments. Technically 3300 feet per segment per the rs-485 standard but you can usually get much more out of it.

[@patrickjones7434]

Hi Phil thanks for posting so much information about bacnet/ip

[@SmartBuildingsAcademy]

My pleasure!

[@DriveYorCareer]

really amazing video sir. thanks for sharing knowledge.

[@SmartBuildingsAcademy]

Glad you liked it

[@brucereed210]

I first heard of "Bam TV " via Republic Broadcasting Network.

[@yateemnk]

Very well explained, thanks

[@SmartBuildingsAcademy]

Glad it was helpful!

[@taskdings]

I learned a lot on this one

[@marcindetnerski4636]

great job!

[@namletranhoai9622]

very helpful! can you show us an example project between two devices on the next video, please?

[@SmartBuildingsAcademy]

Between two BACnet/IP devices?

[@jgillservices]

great video, learnt a lot thanks. i have one question. what would happen if there was no end of line resistor? thanks

[@SmartBuildingsAcademy]

You would have no way of stopping reflections at the end of line and dampening that reflection from causing data degredation.

[@slave288]

While I agree with a lot of your best practices I must point out that there are some things that you said that are completely incorrect. The biggest one being that devices do not have to be addressed down the trunk immediately in order in a sequential addressing scheme. Yes this definitely is good practice and it does make troubleshooting at a later date easier but having out of order devices does NOT decrease bus speed or put any latency on the network what so ever. You are thinking about the network cable more like a car traveling down a road where the token must travel from one device to the next, this is actually not the real case at all. In Network Communications the cable actually acts more like a long tube through which each device listens and speaks to each other through the tube. As the network is all half duplex and is one continuous daisy chain, when an electrical bit signal is pulsed on the network the entire cable must become charged and discharged to signal each bit, think of this sort of like a water hose where the entire hose must be pressurized and depressurized to make a bit signal...so having the token skip over a few devices to get to the next address does not take any longer at all because the entire network cable must be charged and discharged, its all virtual the signal does not actually "travel" down the cable. Now having said what I did above there is one IMPORTANT thing to consider and this one is key. If you have gaps in your addressing sceme, where addressing goes from 1, 2, 3, 4, 5 and then suddenly jumps to 50, 51, 52, 53 then yes this large gaps can and will slow down your network and put a lot of dead time on the network. This happens not because the taken falls off the network or can't travel to the missing devices but because the last Master device must sit and wait a required length of time for an answer before it says to itself "Well that device must be offline, so I'll move on the to the next one." and then tries the next address to pass the token to until it eventually gets an answer and passes the token. So having sequential addresses is highly important, but they do not have to be in order down the wire. Also, while we are on this topic, setting the proper Max Master settings is also important, else when the token gets to the last device on the bus it will just keep calling blank addresses until 127 before it finally starts back over at 0 and works its way back up. Next. I don't know what part of the world you are in, I'd suspect somewhere up North but here in my area we could not even begin to pay an electrical contractor to install controls...they would just laugh as they walked away. Hell we cant even get our standard HVAC technicians to deal with controls...when they see controls on a unit they instantly lock up and can't even do the simplest of things like trying to jump out a unit to check its operations on a PM. If you want a properly installed system...you have to install the system yourself with your own team who is properly trained...otherwise, you are going to be spending a CRAP ton of time going back through every bit of the system trying to determine what they did incorrectly and why the system won't work at all. When it comes to baud rates...I guess you must be a JCI Rep? Doesn't matter but there are a lot more speeds than just 38400 and 9600 and if you're going to talk about Bacnet on a non-brand specific basis then you really should talk about all the speeds and how one should go about selecting and downgrading speeds in a good practice basis. One of the key things I think you left out of your discussion about network practices and common problems is Biasing. Remember that each segment must not only have proper EOLs but also proper biasing.

[@SmartBuildingsAcademy]

9600 and 38.4 are the only two speeds required to be supported by the standard thats why i talked about them. The addressing being sequential does have an effect because of the reason you mentioned waiting on the poll prior to passing the token from master. If you have your addresses out of order this will have an effect on the comm speed over a long 5k feet plus run. Tested this out with wireshark in the lab and proved it out several times. Biasing is important but its rarely a reason i see for trunks going bad. Thanks for taking the time to reply. :-)

[@AgentTrainingOne]

Why do some BACNET MSTP networks use three wire 1.5 TSP? and how do you determine if you need pair wiring vs 1.5 TSP wiring?

[@jamesvanconey6092]

Phil, in order to have more than 32 devices on a trunk do you need a repeater to go to more than 32 (ex. 64 devices)

[@brhanuketema8295]

great

[@jamesvanconey6092]

phil, can you set you max master on all devices to 127 and set EOL to a # slightly higher than your max # on the trunk?

[@CesarPalacios-ld4fq]

on the trunk per Feet limitations is it 1,500 hundred ft or 15,000 thousand ft ?

[@SmartBuildingsAcademy]

15k for full trunk which would be (3) 5k foot segment per ASHRAE 135. R-485 Standard is less than that.

[@JohnnyJr396]

I wouldn’t have though duplicate Mac ids are that common since in theory they’re burned in from the manufacture and should be unique. Are there some cases where they can be changed by a tech or something?

[@SmartBuildingsAcademy]

Yes MAC addresses with ethernet are "burned-in" they actually can be changed though. But MS/TP are usually set via dip switches on the physical controller. MS/TP is a different protocol than Ethernet.

[@JohnnyJr396]

@@SmartBuildingsAcademy many thanks!!

[@swapnilzaveri4271]

When my device is connected in trunk device gets slow. and token get loss any solution for it

[@sakthiprakash5607]

Can i take data from BACnet devices with help of microcontroller & how to take please explain that

[@SmartBuildingsAcademy]

Using BACnet/IP you can get data from most systems.

[@Irnotslowable]

Phil, I have a question about a project we are working on ..BACnet routing - BBMDs, traffic routing/polling via MSTP vs IP, how MSTP serial links function, etc. When there is routing through MSTP-to-IP converters (such as supervisory controllers or 3rd party devices like those from LOYTEC and Contemporary Controls), is the polling interval controlled by the MASTP routing/converting device or does the polling IP device control the poll rate of the serial link? Also, if controlled by the polling device, can each device on a link be polled independently at a different rate? Or must all devices on the link be polled at the same rate? Let me know if you have any questions about this. If you need to email me please do so at frank AT devicoservices DOTT COMMM. Thank you

[@howardsanford5550]

You need to show a better view on the board. You got glare plus you are using black marker. The lighting is very bad.

[@SmartBuildingsAcademy]

Yes it is, that's why we stopped our videos for a bit and have upgraded our lighting and video systems.

[@MachChicken]

@@SmartBuildingsAcademy just watched, no complaints.