Bacnet MS/TP - Master vs. Slave

[sysint]

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.

Picnik... you should know by now I'm fully aware of this already. As long as you brought it up - Somewhere Siemens wrote a nice article (I'd like to find again) showing how much better this solution is compared to MSTP.

[s2sam]

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.

Good day Sysint,

Indeed, in regards to my comment about the Neuron chip's possible future issue(s) the risk is small, but nevertheless there. Where this differs from BACNet (assuming one implements their own BACNet stack) is that if there is a protocol issue (i.e. bug) a manufacturer has the ability to sort it out...as the the problem can be corrected in firmware (assuming the problem is not a faulty hardware design). Using a Neuron chip (or a dedicated third-party BACNet chip), the manufacturer is hooped and at the mercy of the Neuron (or BACnet) chip manufacturer to fix the problem. Secondly, if it is shown that the chip itself has issues, then the only solution is to replace the chip... which is not very simple or economical

Now, to be fair there are other single sourced chips in the manufacturer's design... such as the microcontroller or microprocessor, however, because these chips are generic they are sold in much higher volumes (100's of thousands, millions, etc) and so major bugs within them would have been resolved fairly early on in their release.

I make these points as items to be aware of and not meaning to slight any one vendor or protocol.

Cheers,

Sam

[sysint]

Well yes I suppose. However, you can implement LON off the chip. - Anyway, it's not like there is one way to do this. However, this one is rather inexpensive.

[AbeS187]

Good day Sysint,

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.

The full-load (32 device) transceivers are more noise-immune than the 1/4 or 1/8 load RS485 transceivers. VFD's often use the full-load transceivers, because of this. A 480VAC VFD has about 600VDC on their power circuitry which is physically located close to the isolated RS485 circuits. In many cases it is not about saving a few pennies, it is often about the quality of communications.

[s2sam]

The full-load (32 device) transceivers are more noise-immune than the 1/4 or 1/8 load RS485 transceivers. VFD's often use the full-load transceivers, because of this. A 480VAC VFD has about 600VDC on their power circuitry which is physically located close to the isolated RS485 circuits. In many cases it is not about saving a few pennies, it is often about the quality of communications.

Good day AbeS187,

I do not know if you can say this across the board for all 1 Unit Load (UL)RS-485 transceivers. By definition a RS-485 transceiver must comply with the minimum requirements which are set out in the electrical specification(s) for the RS-485 (or should I say EIA-485) devices. Some manufacturers exceed these minimums and some offer devices which guarantee these "enhanced" specifications... but not all. Remember if the specs are not "guaranteed" then you cannot assume that these enhanced specs will always be available as these specs can and will change on the respective manufacturing run. For specific design critical specs I always qualify a manufacturer and I will not allow a alternative manufacturer's component substitution unless I have qualified this new vendor's part. Sadly, at times purchasing department personnel see all components as generic and as such feel they can interchange parts (assuming functional/footprint compatibility, etc) based upon pricing... this is indeed not so on a number of components that are deemed critical to a circuit's functionality and reliability.

Another item which may be related to your comment is that the 1 UL devices have a lower input impedance (minimum of 12K ohms by spec) than the 1/4 UL (48K ohms) or 1/8 UL (96K ohms) devices which in itself may offer less susceptibility to noise.

Cheers,

Sam

[AbeS187]

Good day AbeS187,
Another item which may be related to your comment is that the 1 UL devices have a lower input impedance (minimum of 12K ohms by spec) than the 1/4 UL (48K ohms) or 1/8 UL (96K ohms) devices which in itself may offer less susceptibility to noise.

The lower input impedance results in pulling more current from the bus hence it is generally MORE immune to noise.

[justjohnson]

MORE immune to noise. less susceptibility to noise.

Aren't these two statements the same?

[AbeS187]

MORE immune to noise. less susceptibility to noise.

Aren't these two statements the same?

Yes, I must have misread that. Sorry about that!

[s2sam]

The lower input impedance results in pulling more current from the bus hence it is generally MORE immune to noise.

Good day Abes187,

The Input impedance by itself will not always guarantee higher immunity to noise. Remember that RS-485 is a differential signalling medium and as such common-mode noise will be cancel out... i.e. the same noise (assume common-mode) will be present on both terminals independent upon the input impedance and therefore will be canceled out... This is assuming, of course, that cabling is correct for RS-485 (suitable twisted pair, daisy chaining, etc).

Cheers,

Sam