Onicon System 10 MS/TP issues on FX60

[DST-Tech]

We have one of our customers where we are setting up some Onicon System 10 BTU meters. They are on a separate stand-alone loop right now and when we connect them to the existing FX60 BACnet MS/TP trunk, it causes some of the other controllers to randomly go online and offline. We did discover a duplicate address on one of the meters and removed it. However, right now we can only leave two of the meters connected, because any more than that causes instability. I am not sure if it is simply the number of meters on the trunk. They all can't be bad. We still have to do more testing, but was wondering if anyone has encountered communication issues while using this hardware. A couple of days have already been spent on something that shouldn't take this long. I can scan the meter loop alone and see all of the meters, but as soon as we connect them to the bus, everything goes nuts. As a last resort, I have thought about placing them on their own dedicated loop, but this will incur more expense and I'm not sure if this will cure the problems.

[Igotworms]

Is there a duplicate instance number?

[DST-Tech]

I don't think so. As each device was addressed, the "Device ID" was also set to the address, but in the 57000 range. So, address 16 is 57016, 17 is 57017 and so on. I am assuming that this relates to the instance. I do not see any other data fields that relate to device instance. Thanks.

[kc2dnw]

Couple things to check

1) your new devices, the MAC address for each device is unique on the wire you are connecting it to correct?
2) Your new devices, the instance (Device ID) is unique across your entire facility of any BACnet device?
3) The current MS/TP network you are adding these devices to, is there more than 32 electrical loads on the network? If so did you add a repeater after the 32nd electrical load on the network?
When you scan the bus I assume you are using some sort of BACnet software and are all the MAC address and instances showing properly? Did you do a scan of the existing bus to verify no duplicates? This sounds exactly like a duplicate address issue.

-Jeremy

[DST-Tech]

Items 1 & 2 are OK. I did not realize that a repeater would be needed for more than 32 devices. Right now, the network seems stable with 31 controllers on it. That would make sense, because we had to remove meters until it finally stabilized, and we are currently at 31 devices connected. Any repeater that you would recommend? Thank you.

[s2sam]

<snip> I did not realize that a repeater would be needed for more than 32 devices. Right now, the network seems stable with 31 controllers on it. <snip>

Good day DST,

The number of 32 must be taken in context. In general terms RS485 networks like BACNet MS/TP, N2, etc is defined within the RS485 (actually EIA-485) specification to be 32 unit loads (UL)... Older or standard RS485 transceiver devices are rated at 1 UL and thus 32 would be the maximum. However, there are newer and better RS485 devices and these are rated at 1/4 and 1/8 UL and so you can place more devices (4 or 8 times respectively) on a RS485 bus without violating the spec... What happens when you exceed the spec? This will be very site and controller specific (i.e. some controllers have self termination, bus biasing, etc which affects its input impedance), but in general you start to widdle away at the bus's noise immunity and so you can get random offlines, data transmission errors, etc... basically a flaky bus.

Cheers,

Sam

[DST-Tech]

This particular trunk is also pretty long. The FX60 runs the entire building, 45 stories to the top; all of the plant equipment below grade, plus a few small AHUs, then the towers at the top of the building. All the stuff in between is 2 pipe standalone control. But, the bus is run out all over the place. Thinking about it more, I am probably better off separating the meters off and put them on their own trunk. The rest of the building works fine and I would have to place a repeater somewhere in the network, so I will still have to run more wire anyway. A little food for thought.

[nasaguyjoe]

Did you ever come to a solution on this? I'm running into a similar problem. Have a trunk with just 5 meters and after a while (days) they will all go offline. Then I have a trunk that has various devices on it and after a few days they will all go offline. I'm well under 32 devices in all cases.