S2USB485

If there has been a history of devices going off line in the pure Metasys environment it is a sign that something is wrong.

• It could be bad wiring,

• it could be too many N2 devices on the bus,

• it could be EMF,

• it could be a bad transmitter on a N2 device,

• it could be multiple masters accidentally installed on the bus.

Determine the issue and correct it before proceeding with the integration.

You can use the ComBus Quick Tester, a digital volt meter, or similar tool to test the N2 bus. In order to perform the test you need to disconnect it from the supervisory controller. If using the ComBus Quick Tester you simply plug the Phoenix Contact connector into the tester and start the process. If using a digital volt meter you need to measure the voltages between each of the legs of the N2 bus.

According to the documentation that comes with the ComBus Quick Tester the guidelines for a successful test are:

• to Ref VDC 2.45 -3.00 will pass

• to Ref VDC 1.90 - 2.54 will pass

• to - VDC 0.38 - 0.92 will pass

We've seen multiple areas where improvements can be made in existing N2 buses. From an electrical point of view the N2 bus follows the RS-485 standard with a N2+, N2-, and reference line. In many cases we will point you back to JCI documentation for details. We're not going to try to re-engineer the N2 bus technology. It is what it is.

We have seen many cases where either improper installations or just normal wear and tear over the years has taken its toll. It's also a fact that once you integrate to a legacy N2 bus with either the OPC-N2 Router or the BACnet-N2 Router it will be driven harder than it has ever been driven before. If anything is marginal, it will be exposed by the additional load put onto the system.

We always advise our integration partners to validate the N2 bus as part of determining the scope of an integration project. In fact, we advise them to specify in their contract that the customer deliver to them an in-spec and working N2 bus with wo...

It is very important to validate that the N2 bus is within specs as part of the planning for a N2 integration. The N2 bus is based on the RS-485 specification.

Some JCI N2 devices have a termination switch, others have a self termination capability.  Research the JCI documentation carefully before assuming that the end of each N2 bus segment needs termination.

When you remove a legacy supervisory controller from the N2 bus it may have been providing termination that needs to be replaced to keep the bus operating within spec.

The more information you have at hand the less risky a N2 integration will be and the more successful the project will be. In preparation for a N2 integration we always recommend that you:

• Use HVAC Pro and create a Network Map report with it. This will tell you the hardware type, Firmware versions, and application loaded into each N2 device.

• If you see any Generic IO / Point Multiplexor applications this is a red flag. That means that the logic driving those points is back in an NCM, or other supervisory controller. You can still do a very successful integration. You just need to do more planning.

• Make sure that each N2 bus segment is within electrical specifications. You can use the ComBus Quick Tester or a DVM. You will be driving the N2 bus harder than it has ever been driven before so any weakness will come out.

• Review as built drawings and any other documentation that is available for the Metasys installation. I've taken the approach of s...

When we designed the S4 Open Appliances we determined that we would stop short of providing these services. Johnson Controls has very effective tools for performing these functions that are aware of all of the variations in firmware, application revisions, etc. We didn't feel that we could add any value in this area.

Use HVAC Pro for N2 Open devices like the UNT, VAV, and AHU

Use HVAC Pro for the VMA

Use GX-9100 for the DX-9100

One of the things that we did include in the design is the ability to easily determine the application loaded into N2 devices, the firmware version running in the device, the original configuration file name, etc. This information is provided on the properties page for each device in the Building Control Network, BCN, folder. The information provided by each device type varies. Details for many N2 devices are provided now. Others will be added in the future as time and resources allow.

The OPC server associated with the S4 Open: OPC-N2 Router provides a Quality ;attribute for each point that it publishes. This indicates if the S4 Open Appliance is successfully talking to the field devices and retreiving data points. Unfortunately, some OPC clients ignore the Quality indicator and instead default the data values to zero.

The S4 Open Appliance will retry transactions to a N2 device multiple times before it gives up and returns a BAD Quality indication.

Another symptom of this same problem is that the Configure Wizard fails to find all of the N2 devices on the bus. If the Configure Wizard does not find a device that you know is on the N2 bus that tells us that there was interference during the time that we were trying to access N2 address XX (in hex). It is not necessarily a problem with the device at this address. The Wizard simply saw the bad transmission in the time frame that we were expecting a response from this device.
...

The Metasys N2 bus follows the RS-485 standard for its physical and electrical characteristics. It runs at 9600 BD, half duplex using N2+, N2-, and Ref wires. Our S4 Open appliances are pre-configured to have all Com ports operate in RS-485 mode.

If you run any of the JCI Metasys configuration or commissioning tools HVAC Pro, GX-9100, etc. from your labtop or another Windows PC and want them to communicate the the N2 bus via their COM port you'll have to utilize either a USB to RS-485 or a RS-232 to RS-485 converter.

We have been using RS-485 converters from B&B Electronics www.bb-elec.com to connect our PCs to the N2 bus.

For RS-232 to RS-485 we use their model 485SD9TB

For USB to RS-485 (N2 bus) we use their model number 485USBTB-2W

The USB Serial Port technology has become so standardized that most manufacturers' products should work fine although the B&B products are the only ones we have tested. Note that you may have to a...

The JCI design of the N2 bus is fixed at 9600 Bd. We have to live with that. It can’t be changed.

So, the problem becomes how do we accommodate BACnet IP or OPC transactions coming in at Ethernet speeds?

• Transparent Caching and BACnet COV support improve efficiency

• Validating the N2 bus, repairing any installation problems, and removing sources of electrical interference improve performance

• Breaking the N2 bus into smaller segments distributes the load. Each segment may come into a separate Downstream N2 Interface on a N2 Router or each one come into a separate N2 Router

• Poll rates may need to be adjusted to frequencies more realistic for the capabilities of the N2 bus