Obvius BACnet - Manual (Page 46)

BACnet Server for AcquiSuite Page 46 of 52
CVS:bacnet/Documentation/BACnet_Users_Guide.doc
8.7 Symptom: Relays Turn On When They Shouldn’t?
This section deals with the symptom of relays turning on when they shouldn’t, or turning on seemingly at
random, or appearing to have “glitches”.
All of these symptoms result from lack of fail safe logic design.
“Fail safe logic design” means designing your control logic so that if communication with a sensor fails
for any reason, the logic outputs (to relays) go to the inactive (off) state, or at least remain unchanged.
The easiest way to ensure your logic design is “fail safe” is to test it by physically disconnecting
sensors and observing the result.
Even in a properly working system, momentary communications failures are inevitable for these reasons:
1. The AcquiSuite gives priority to logging of data over responding to BACnet requests. At the start
of each data logging cycle, the AcquiSuite will pause in servicing Modbus read-requests for
approximately 10 seconds.
2. The response time of the AcquiSuite varies with the “load” on the RS485 bus, and this load may
vary depending on the number of requests received over the network (from BACnet, from XML,
from Modbus/TCP, etc.)
3. Electrical noise (such as lightning strikes) which disrupt Ethernet routers or switches or RS485
buses.
4. By default, the AcquiSuite checks for firmware updates. Installing an update means the
AcquiSuite will reboot, causing a delay of approximately 1-2 minutes.
You can deal with these issues by,
· Properly setting your Controller’s “Stale Timeout” (if it supports such a feature),
· Having your Controller monitor the Status_Flags and Reliability properties on objects shared by
the AcquiSuite.
· Incorporating both of the above into your control logic.
8.7.1 Setting a “Stale Timeout”
How you set a “stale timeout” is Controller-specific. For instance, the Tridium JACE has a “Stale Time”
parameter in its BACnet Tuning Policy, which sets the maximum response delay before a data point is
considered “stale” (see below). This should be set as follows: