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Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

Archives: Building Automation and Control

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sesa125819
Schneider Alumni (Retired)
Schneider Alumni (Retired)
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Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

A customer wishes to use this configuration, is it possible?  If not, then why is it not possible.

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Jeff
Administrator Administrator
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Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

I perceive three big potential problems:

  1. Are you planning on learning the devices in beneath both Automation Servers?  If they are in the same system, I'd expect you'd have trouble learning each in beneath each AS because you'd get Duplicate BACnet ID errors.  I'd also suspect you'd be forced to use different Network IDs if in the same system (because I recall the system checked for duplicate Network IDs before letting you set one).
  2. If these are devices that are natively programmable within SBO, I'd expect some serious headaches.  Whenever you make changes to objects under one AS, you wouldn't automatically see the changes under the other AS.  In the case of b3 controllers, there is no way to save to database or sync objects back in from the device.
  3. I would expect possible issues when broadcast messages are sent with a destination network of 65,535.  Though I'm not an expert on the subject, I expect they might get in a routing loop.  For example, the message is generated by a device on Network ID 1.  One or both of the Automation Servers then re-generates the message on the MS/TP bus, but when each hears a message on the MS/TP bus that was originated by the other and sees the desination network of 65,535, they would be compelled to re-create the message on Network 1, at which time the other AS might hear the message and assume it needs to be replicated on the MS/TP bus (and so on).

If you're able to make it work cleanly, I'll be very interested to hear the details of how you pulled it together.

See Answer In Context

7 Replies 7
sesa139513
Schneider Alumni (Retired)
Schneider Alumni (Retired)
0 Likes
3
68

Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

Hi Mike,

Two AS's on one BACnet MSTP is possible. The next question would be why? I am sure the site has some good reason for wanting to configure this way, but with Lessons Learned article 7070 it shows the limitations of BACnet devices for 1.3 is still at 50, so why use up one of those with another AS? Suggest perhaps, that the capabilites of the AS with BACnet/IP is much more robust and with both channels, (A&B) they can have BACnet on both. Be reminded that this still limits to 50, which would be 25 devices on each channel.

Best regards,

Houston

sesa125819
Schneider Alumni (Retired)
Schneider Alumni (Retired)
0 Likes
2
68

Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

Thanks for that Houston, I have still to find out what the customer is trying to do. If he did use two AS's, how would they be configured?

Julio_Cortes
Commander Commander
Commander
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0
68

Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

Mike,

I think it is also going to depend on what type of BACnet devices will be part of the topology. BACnet devices can act as clients and servers, the next question I would ask if both of the Automation Servers will be requesting data from the same BACnet devices?  Can the BACnet devices support more than one client?

sesa139513
Schneider Alumni (Retired)
Schneider Alumni (Retired)
0 Likes
0
68

Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

BACnet2AS.png

In the simplest form of BACnet, it is just a 3rd party BACnet device. They would be configured like any other BACnet device on the MSTP network. This screen shot is an AS with BACnet Interface, then an MSTP that has an ASNumber2 shown, and a AS2BACtemp on the AS which is just a value on that second AS. Configuration Settings to consider would be Network ID, RS-485 Port, MAC address, and MSTP Baud Rate. This would be for a simple b3 and AS on MSTP. Julio is correct in that if it is more entailing, then find out the details to take into consideration.

Jeff
Administrator Administrator
Administrator
0 Likes
2
69

Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

I perceive three big potential problems:

  1. Are you planning on learning the devices in beneath both Automation Servers?  If they are in the same system, I'd expect you'd have trouble learning each in beneath each AS because you'd get Duplicate BACnet ID errors.  I'd also suspect you'd be forced to use different Network IDs if in the same system (because I recall the system checked for duplicate Network IDs before letting you set one).
  2. If these are devices that are natively programmable within SBO, I'd expect some serious headaches.  Whenever you make changes to objects under one AS, you wouldn't automatically see the changes under the other AS.  In the case of b3 controllers, there is no way to save to database or sync objects back in from the device.
  3. I would expect possible issues when broadcast messages are sent with a destination network of 65,535.  Though I'm not an expert on the subject, I expect they might get in a routing loop.  For example, the message is generated by a device on Network ID 1.  One or both of the Automation Servers then re-generates the message on the MS/TP bus, but when each hears a message on the MS/TP bus that was originated by the other and sees the desination network of 65,535, they would be compelled to re-create the message on Network 1, at which time the other AS might hear the message and assume it needs to be replicated on the MS/TP bus (and so on).

If you're able to make it work cleanly, I'll be very interested to hear the details of how you pulled it together.

Julio_Cortes
Commander Commander
Commander
0 Likes
1
68

Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

I think we have to many unknowns, but I would think you can manually assign the Instance of each AS and disable the IP network to prevent routing issues, if they are only going to be part of the MSTP network.   We do this all the time with MNB-1000s, UNCs, ENCs.

sesa125819
Schneider Alumni (Retired)
Schneider Alumni (Retired)
0 Likes
0
68

Re: Can a SmartStruxure BACnet MSTP lan have two AS's connected to it?

Thanks to everyone for joining in on this, here is some information from the customer.

Because we are testing AS BACnet MSTP with Honeywell controllers in a
different network. However, we have some problems for the first test
communicating with their controllers. So we have an idea or option of
testing if we can put one AS in their network and talk to our AS through
BACnet MSTP. We are not sure about if our AS can used for slave or not,
or our AS is only used as Master?