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  1. #1
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    Max Masters

    I was under the assumption that the max masters setting referred to the physical number of devices on a network but as I sit here after wasting a day trying to get a bacnet network communicating it occurred to me that the max master setting may refer to the max MSTP address on the network which would explain why I can't discover some devices and have iffy comm. throughout my 5 networks. Can someone confirm or deny. I'm not looking to start another lon vs. bacnet debate.

  2. #2
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    max master = max mstp address, can go up to 127.
    this is for token passing and auto discovery in BACnet.
    A BACnet slave device is a different beast and can have mstp address beyond 127 but it is unlikely that you have slaves, you probably only have masters.
    good luck.

  3. #3
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    Quote Originally Posted by amigo View Post
    max master = max mstp address, can go up to 127.this is for token passing and auto discovery in BACnet. ...
    each Bacnet MSTP device is token passing .
    it gets the token...talks some crap (lots of crap actually. Hmmm, prolly too much crap) and then passes the token on to another MSTP address.
    not 100% sure of the precise protocol involved but it probably gets to the highest addressed device on your physical bus like MAC=50...
    and then 50 goes...51 you there, I got a token for you?...psst 52, you there? ... 53...54?? anybody? ... and then there is some sort of reversion and device 0 says oh geez guys ... Ill just start at 1 again.
    (or something like that!)

    Set yr max master...(hmmm, yes set it on each device and the Workstation) ... to just over your highest MAC address. start yr MAC addressing at the low end rather than from 127 down. zero is normally reserved for the master controller/router/workstation/jace/whateverYourBacnetThingIsCalled

    more idiocy brought to you by the land of BacNet and our friends at ashrae
    Double your IQ or no money back...guaranteed.


    ...I'll prolly get hosed for trashing MSTP ... but man....I DO HATE IT SO MUCH!
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  4. #4
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    Max Master is a MSTP data packet management concept that has been around long before BACnet was ever developed.

    A very common cause of EIA 485 MSTP (Master-Slave-Token-Passing) discovery failure, is that one or more of the devices
    (Masters) on the MSTP Buss, has their internal "Max Master" setting at a number that does not match all of the other device
    "MaxMaster" settings. If you want effective token passing, all devices on the buss need to have the same
    Max Master value put into their configuration settings.

    Many manufacturers ship their MSTB devices with the default Max Master setting at 127 (the max number for the MSTP
    device addresses on a MSTP buss). If you have only 10 devices on you buss, this default setting of 127 can slow you buss down.

    I have seen some "out of the box" MSTP devices that have it default at zero. This will cause problems.

    I would say, give a check of all you MSTP devices on you buss to make sure they are all set to
    the same number, and that Max Master number would be the highest address plus 1.

    If you want a faster buss, keep all you addresses in order, without gaps (1,2,3,4,5,6, etc)

    Hope this helps.

    By the way, BACnet did not design or develop any of the electronic comm busses that their BACnet data packets
    and protocol ride on. They just created data sharing rules that allow BACnet data packets to co-exist within
    the minimum functional requirements of the underlying electrical buss requirements.
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

  5. #5
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    Quote Originally Posted by dracula View Post
    By the way, BACnet did not design or develop any of the electronic comm busses that their BACnet data packets
    and protocol ride on. They just created data sharing rules that allow BACnet data packets to co-exist within
    the minimum functional requirements of the underlying electrical buss requirements.
    Au contraire Monsieur Dracula,
    the underlying bus is nothing but 2 wires....the rest was definitely designed by 'committee'.
    The most annoying thing is having to explicitly set things like MM and MAC and even DevID
    ...that a clashing MAC will render the physical medium useless, why design a system where the MAC can clash and render the whole bus redundant. Its just silly.
    ...the ONLY way to find a clash is to either split the bus into smaller and smaller segments, OR visit every controller in turn with the laptop...again just silly
    ...Why enter MM at all? I should think a device would announce it self using part of the protocol that isnt MAC reliant, and then join the token-passing schema in an elegant way. Like merging into traffic.

    Bacnet has some great features but Im afraid MSTP isnt one of them.

    Polling at 78k would be near as fast for practical purposes as token passing...and no clash problems ... again just silly

    Quote Originally Posted by dracula View Post
    A very common cause of EIA 485 MSTP (Master-Slave-Token-Passing) discovery failure, is that one or more of the devices
    (Masters) on the MSTP Buss, has their internal "Max Master" setting at a number that does not match all of the other device
    "MaxMaster" settings.
    Pffft ... device in the bus that is getting messages to talk crap then pass the token...so that the rest of the bus can function ... and um, its just gonna sit there like a mute ... just silly

    at least this thread will make people think about and understand some of the settings required .... wouldnt it be a lovely world if MSTP delivered on the promise to be simple, straight forward, fast like it promised

    Quote Originally Posted by dracula View Post
    I would say, give a check of all you MSTP devices on you buss to make sure they are all set to
    the same number, and that Max Master number would be the highest address plus 1.
    LOL ... step 1 ... check each device and make sure the MAC and DevID are DIFFERENT.
    Step 2 ... check each device and make sure the setting for MM in each device is the SAME.
    Step 3 ... wait and see.

    that definitely sums up MSTP!
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  6. #6
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    Quote Originally Posted by MatrixTransform View Post
    Au contraire Monsieur Dracula,
    the underlying bus is nothing but 2 wires....the rest was definitely designed by 'committee'.
    The most annoying thing is having to explicitly set things like MM and MAC and even DevID
    ...that a clashing MAC will render the physical medium useless, why design a system where the MAC can clash and render the whole bus redundant. Its just silly.
    ...the ONLY way to find a clash is to either split the bus into smaller and smaller segments, OR visit every controller in turn with the laptop...again just silly
    ...Why enter MM at all? I should think a device would announce it self using part of the protocol that isnt MAC reliant, and then join the token-passing schema in an elegant way. Like merging into traffic.

    Bacnet has some great features but Im afraid MSTP isnt one of them.

    Polling at 78k would be near as fast for practical purposes as token passing...and no clash problems ... again just silly



    Pffft ... device in the bus that is getting messages to talk crap then pass the token...so that the rest of the bus can function ... and um, its just gonna sit there like a mute ... just silly

    at least this thread will make people think about and understand some of the settings required .... wouldnt it be a lovely world if MSTP delivered on the promise to be simple, straight forward, fast like it promised



    LOL ... step 1 ... check each device and make sure the MAC and DevID are DIFFERENT.
    Step 2 ... check each device and make sure the setting for MM in each device is the SAME.
    Step 3 ... wait and see.

    that definitely sums up MSTP!


    Please read step one above. Now that you have read it I have a question.

    Why does the Mac and DevID HAVE to be different.....

    I like to put the MAC address as the last two numbers of what I use for a DevID.

    Are you saying that if the MAC is 12 then the DevID needs to be something other then 12?
    UA Local 141

  7. #7
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    Quote Originally Posted by mechmike2 View Post
    I'm not looking to start another lon vs. bacnet debate.
    Like OP says, let's stick to answering his question.

  8. #8
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    Quote Originally Posted by amigo View Post
    Like OP says, let's stick to answering his question.
    well mate ... in a round about way it is answering his question innit?

    if the process and pitfalls are understood then its easier....gotta know the beast to kill the beast.

    sorry if compared to LON that MSTP looks stupid...not my problem.
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  9. #9
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    I do prefer lon but you gotta do what you gotta do.

    How about the max info frames, is there a different setting for each manufacturer?
    I have been trying to find a spec to determine what that should be set for but all I get is leave it at default.
    For a York RTU the default is 1 which seems to me would cause tons of traffic. Carrier RTU Open defaults to 10. Honeywell Spyder defaults to 20. Functional Devices bacnet relay default to 1. I would think the more info a device can transfer while it has the token the better the network as a whole would operate, but finding the pertinent information seems difficult.

  10. #10
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    Quote Originally Posted by mechmike2 View Post
    ... How about the max info frames ... finding the pertinent information seems difficult.
    Now there is a question that Id like a succinct answer to as well ...

    and what about APDU timeout ... what is that exactly?

    anyone??
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  11. #11
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    Quote Originally Posted by MatrixTransform View Post
    Now there is a question that Id like a succinct answer to as well ...

    and what about APDU timeout ... what is that exactly?

    anyone??
    I believe is has to do with how the controller responds. Some third party devices take longer to respond than others. I've had problems with controllers going on and off line randomly, all I had to do was increase this time. Its just another variable to make the ms/tp trunk even more vulnerable than it already is.

  12. #12
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    nice brief doc from Carrier... http://www.xpedio.carrier.com/idc/gr...808-417-01.pdf

    MM looks like a way to not waste time looking for nodes that arent there...
    APDU looks like how long to wait before deciding to move on because on no answer...
    Max INFO frames seems like a limit on how much crap an MSTP device is allowed to talk before being compelled to pass the token.

    Heres the thing ...Like every polling or master-slave or token passing bus, the physical medium must be solid or the whole system is going to fall down and not work
    assuming the physical is correct and the comms is robust it makes me wonder WHY arent things like MM set automatically by the network?

    Adding a new MSTP device should beautomatic in my opinion too. Considering that there is already a net-wide unique devID... should be able to hook it on the bus...everybody goes 'whoa dudes...new device here" and comms moves on...in fact why have a MAC at all?? why not shunt around the whole devID its only 2 or 3 bytes bigger than the MAC...ah, but of course because somebody in the committee needs a way to ID a device on the bus with a DIP switch ... so MSTP needs just one more little address space.
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  13. #13
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    Quote Originally Posted by mechmike2 View Post
    I was under the assumption that the max masters setting referred to the physical number of devices on a network but as I sit here after wasting a day trying to get a bacnet network communicating it occurred to me that the max master setting may refer to the max MSTP address on the network which would explain why I can't discover some devices and have iffy comm. throughout my 5 networks. Can someone confirm or deny. I'm not looking to start another lon vs. bacnet debate.
    What kind of controllers are you using? I just recently had a similar problem and it ended up being controller specific...


    Im like a mushroom, they keep me in the dark and feed me crap.

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