Page 1 of 2 12 LastLast
Results 1 to 13 of 22
  1. #1
    Join Date
    Nov 2006
    Location
    New Hampshire
    Posts
    255

    Bacnet MS/TP - Master vs. Slave

    Can someone help me understand a little bit better MSTP slave devices? I understand they do not particpate in the passing of the token and therefore cannot initiate information requests, only respond to masters request for info from them. From what I gather most are also only 1/8 load transceivers so you can add more than normal on the bus.

    What I am looking for is:
    1) What other devices on the network do you need to communicate to them - I have seen some references to a Master proxy - what is that?
    2) How many devices can you realistically put on one trunk - the max number of masters is 124 but slave devices can exceed that?
    3) What is the impact on polling rates/baud rates/performance, etc. when adding slave devices to a MSTP trunk.
    4) Is anyone using them extensively? Most of the bacnet devices I come across are masters on the MSTP network.

  2. #2
    BACnet is offline Removed by Admin- No Return
    Join Date
    Aug 2009
    Posts
    2,459
    JJ- Those are some good questions. Let's see if I can shed some light on them for you.

    1) You can communicate with a slave device just as you would a regular master device in as much as you can read and write points. What makes a device a slave device is the fact that it's not in the token ring, nor can it initiate any messages- only respond to them.

    Some master devices can be set up to be proxies for slave devices in BACnet as you have mentioned. These controllers respond for the slave device in certain instances, as though they were the device itself. An important function that a proxy would do would be responding to a who-is for a slave device so that front end thinks that the device is present, alive, etc. I personally don’t use this functionality much myself but there are a number of controllers that can do this for you if you require it. (It is my contention that you likely don’t need this feature, just poll for the data directly.)

    2) Technically speaking, a trunk can have up to 255 (0 through 254, the number 255 is wild) devices on the trunk. The number of masters is limited by the range that is applicable to masters, so only 128of them (0 through 127) may be considered masters. Please note that this doesn’t mean that a device in this range has to be a master, it can also be a slave assigned an address between 0 and 127…

    The question of “realistic” numbers of devices that can be on one trunk is a bit harder to determine. Every manufacturer has a different rule of thumb and in reality it has to do not only with the length of the wire and the quality of the wire but also the overall impedance of each of the devices attached. I have seen some manufacturers suggest that 32 is a good max number, others that suggest 128, etc. Throw repeaters into the mix and this question becomes arguably worthy of an entire thread.

    3) The actual act of adding a slave device does not take up any “time” on an MS/TP trunk. It is invisible to the token passing portion of the communications, so it doesn’t effect the time it takes the token to circle the ring. If you are reading or writing a lot of points on the device then those transactions will take time but no more so than they would take asking the same questions of a normal master device.

    4) A lot of manufacturers allow their devices to be put in slave mode and there are more yet that make devices that can only be configured this way. If you absolutely require the fastest token ring around, you can always limit the devices getting the token which will decrease the time it takes the token to loop the entire ring. I personally don’t think it makes much difference but I’m sure people could come up with oddball examples where it made all the difference.

    In the case of slave-only devices, often this is the only way to make a very limited controller (slow processor, low RAM, etc) exist on the MS/TP trunk. Implementing the entire MS/TP stack is not always an option for say a PIC processor with 4K or RAM…

    I hope this helps!

    -B

  3. #3
    Join Date
    Nov 2006
    Location
    New Hampshire
    Posts
    255
    thanks, I think that gets me going - I am starting to see more and more of these and just need to understand the differences.

  4. #4
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    596
    Quote Originally Posted by BACnet View Post
    <snip>
    also the overall impedance of each of the devices attached. I have seen some manufacturers suggest that 32 is a good max number, others that suggest 128, etc. Throw repeaters into the mix and this question becomes arguably worthy of an entire thread.
    <snip>
    Good day BACNet,

    You are correct here, however, the number of 32 or 128, etc is totally dependent upon which RS-485 transceiver the device manufacturer used. The original and older style RS-485 transceivers had/have an input impedance of 1 unit-load or 12K ohms and was designed to have a maximum of 32 devices (32 unit loads) connected on a bus (RS-485 specification). However, they do make RS-485 transceivers with 1/4 unit load (48K ohm) and 1/8 unit-load (96K ohm) allowing of a maximum of 128 (1/4 unit load devices) or 256 (1/8 unit load devices) respectively. Sadly, each manufacturer uses whichever unit-load device they wish and usually this is dictated by cost.

    What happens if you exceed the 32 unit-load spec? If the device just meets the RS-485 drive specification, then you have reduced noise immunity... which results in a problematic bus and/or reduced bandwidth (because the electrical signals may get corrupted by the electrical noise and so will need to be re-transmitted). The bottom line is you take your chances if you exceed more than 32 unit-load devices on your bus.

    The moral of the story... the RS-485 transceivers used by the manufacturer will determine the maximum (electrical) number that you can have on the bus (max may also be limited by the communication protocol) and still maintain a high quality (high noise immunity) bus.

    Cheers,

    Sam

  5. #5
    Join Date
    Nov 2006
    Location
    New Hampshire
    Posts
    255
    Is there any standard happening out there in terms of transceiver chips being used? Are most slave devices 1/8 load and most master devices 1/4 load? Or is it really something that varies from manufacturer to manufacturer and you just need to make a phone call every time you get a new device to ask?

  6. #6
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    596
    Quote Originally Posted by justjohnson View Post
    Is there any standard happening out there in terms of transceiver chips being used? Are most slave devices 1/8 load and most master devices 1/4 load? Or is it really something that varies from manufacturer to manufacturer and you just need to make a phone call every time you get a new device to ask?
    Good day JJ,

    In regards to the BACNet MS/TP spec itself, I do not recall seeing any particular requirement for the RS-485 transceiver impedance... although perhaps there might be. Perhaps someone who has thorough knowledge on the spec will chime in. I suspect that it is "implied" that a higher impedance transceiver should be used in order to be able to support all of the devices spec'd. So, given this (potential) ambiguity the transceiver's selected will vary from manufacturer to manufacturer and even product to product. Sadly, most products are manufactured with the lowest bill of material cost without regards to consequence... Higher impedance RS-485 transceivers typically cost more (maybe a $1 more) and so are subject to being substituted with the lower impedance (and cost) versions. I have seen some really silly component substitutions and design changes for the simple sake to reduce a bill of material cost by tens of cents for less. Indeed, one must watch the cost of things, but not at the sacrifice of durability, reliability, and quality...

    Cheers,

    Sam

  7. #7
    Join Date
    Apr 2007
    Location
    San Diego, CA
    Posts
    1,322
    Not sure if this is the latest trend, but the new BACnet spyders are 1/4 load devices.

  8. #8
    Join Date
    May 2002
    Posts
    9,564
    Quote Originally Posted by s2sam View Post
    Higher impedance RS-485 transceivers typically cost more (maybe a $1 more) and so are subject to being substituted with the lower impedance (and cost) versions.
    Sam - Are you SURE about that pricing? What total cost are you talking about if they are one dollar more?

  9. #9
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    596
    Quote Originally Posted by sysint View Post
    Sam - Are you SURE about that pricing? What total cost are you talking about if they are one dollar more?
    Good day Sysint,

    I am fully aware of pricing of these transceivers, as I am actively involved in electronic (and software, firmware, etc) design and manufacturing daily. As for total cost, one has the define this accordingly. My comment about a $1 (or so) more was based upon the RS-485 transceiver itself and did not include any additional costs for additional circuitry that may be included (like transient protection, etc). As with all electronics, purchase volume dictates price. So, buying 1 or 2 devices may not translate to a $1 differential... Some of our higher quality (and low impedance) transceivers can cost $6 in small volumes (<10) but are $2 or so in the volumes we purchase.

    Regardless of the cost differential, my point was/is that bill of material cost should not over-ride usability, quality, and reliability. I say this not as just a consumer of these goods, but also as a manufacturer who is truly aware of all of the costs involved and at times annoyed at what short-cuts some manufacturers take.

    Cheers,

    Sam

  10. #10
    Join Date
    May 2002
    Posts
    9,564
    Hi Sam -- I ask because I'm trying to figure out what Echelon is up to because they are going to offer their new Neuron 5000 for $0.60 in quantity and no credits charge for commissioning.

    I'll have to dig. It may be less expensive to run bacnet/LON than MSTP.... and you would get free topology. Further you could probably use LON routing in IP852 and a configuration server. This would make bacnet networks very open and flexible to route with documentation.

  11. #11
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    596
    Quote Originally Posted by sysint View Post
    Hi Sam -- I ask because I'm trying to figure out what Echelon is up to because they are going to offer their new Neuron 5000 for $0.60 in quantity and no credits charge for commissioning.

    I'll have to dig. It may be less expensive to run bacnet/LON than MSTP.... and you would get free topology. Further you could probably use LON routing in IP852 and a configuration server. This would make bacnet networks very open and flexible to route with documentation.
    Good day Sysint,

    I understand. A fairly bullet-proof complete (i.e with transient protection, etc) high impedance RS-485 interface could be built around $6 to $7 or so depending upon volume, etc. I think if you use this number you can determine what the ultimate cost would be.

    As for Echelon's Neuron chip price at $0.60... at what volume are they quoting this at? This price is quite inexpensive considering what the chip does. The issue I would have is if one uses their chip in a design and then sometime later it is discovered their chip has some issues (electrical or protocol), then you are hooped... as you would need to potentially change out the chip in all of your existing installs. I say this not to be alarmist (forgive the pun), but because I know of a site where the Neuron chips had some problems and the result was to replace all of the controllers (maybe 50+)... a number of them were behind finished ceilings, etc...the cost was enormous... and I do not know who actually paid for this repair (customer, manufacturer, Echelon).

    That being said, it is an interesting and potentially cost effective solution all things considered.

    Cheers,

    Sam

  12. #12
    Join Date
    Apr 2008
    Location
    UK
    Posts
    151
    Of topic a little but just for you info Sysint, Siemens Europe does not use BACNet/MSTP for its building controllers but and actually uses BACNet/LON. Its wonderfully easy to install due to the the free topology and I prefer its other BACNet/IP option.

    It is limited to 30 devices per LON segment for some reason but maybe thats so they can sell more routers.
    -----------------------------------------------------
    Don't be a stupid dwarf. It's not big and it's not clever!

  13. #13
    Join Date
    May 2002
    Posts
    9,564
    Quote Originally Posted by s2sam View Post
    Good day Sysint,

    I understand. A fairly bullet-proof complete (i.e with transient protection, etc) high impedance RS-485 interface could be built around $6 to $7 or so depending upon volume, etc. I think if you use this number you can determine what the ultimate cost would be.

    As for Echelon's Neuron chip price at $0.60... at what volume are they quoting this at? This price is quite inexpensive considering what the chip does. The issue I would have is if one uses their chip in a design and then sometime later it is discovered their chip has some issues (electrical or protocol), then you are hooped... as you would need to potentially change out the chip in all of your existing installs. I say this not to be alarmist (forgive the pun), but because I know of a site where the Neuron chips had some problems and the result was to replace all of the controllers (maybe 50+)... a number of them were behind finished ceilings, etc...the cost was enormous... and I do not know who actually paid for this repair (customer, manufacturer, Echelon).

    That being said, it is an interesting and potentially cost effective solution all things considered.

    Cheers,

    Sam
    Sam, I'm saying they offer what looks like a really cheap solution. However, they aren't the only ones able to implement this so I don't think too much issues here to say you are screwed if something isn't right. It's no different than a complete bacnet controller not working really... and I've never heard of a case like you describe, but things can happen to anybody. This gets closer to the D1G model of tossing down bacnet or LON on the same fieldbus wire.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event