Why is my OPC client returning all 0 values for points on some N2 devices?

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.

You can diagnose the problem as follows.

• Start out by validating that the N2 bus is in spec by validating it with the ComBus Quick Tester or a digital volt meter. Wiring problems with the N2 bus could cause signal loss, intermittant connections, data reflections on the wire, and a myriad of other problems. The N2 bus must be strictly a daisy chain topology. There can be no "Y" or "T" branches in the bus. Check for broken wires, etc. In a worst case scenario we have found cases where rodents have chewed the insulation off of the wire and made a nest from it. The bare wires were causing shorts on the bus. As a side note it is perfectly acceptable for the Metasys supervisory controller to be daisy chained in the middle of the N2 bus. It does not have to be at an end of the bus.

• Make sure that there is ONLY one master device on the downstream N2 interface. If you want to simultaneously have your OPC client and your Metasys supervisory controller access N2 data you must utilize the Upstream N2 Interface for the supervisory controller. The S4 Open appliance will act as the gatekeeper to determine when each master device can access the N2 bus. 

- There could be another N2 device with a duplicate of this same N2 address. They will both try to respond to N2 commands at the same time and garble the response.

- There could be another N2 device that is erroneously transmitting on this address. The device might need to be reprogrammed but more likely it will have to be replaced.

• It could be a problem with the N2 device at this address. It could have lost its program, it could be electrically damaged, etc. First try reprogramming it with the Metasys configuration and commissioning utilities. HVAC Pro or GX-9100. These are available through distribution as part of the M-Tools package.

- There could be a device on the N2 bus whose RS-485 transmitter is damaged and transmitting at random times. This device will probably need to be replaced.

 If the problem is that you are seeing zero values through your OPC client you can download either OPC DataSpy ( www.iconics.com) or OPC Explorer ( www.matrikonopc.com) and run it against the current system. Both of these OPC browsers are free downloads. Both of them show the OPC  Data quality indicator along with the data values for each point. This could give us more visibility into what is happening on the bus.

If all else fails give us a support call and we can assist you with capturing the activity on the bus with a serial monitor and interpreting the results.