Product manual

B Protocols TAC Xenta Server – Gateway, Technical Manual
164 (184) Schneider Electric Buildings AB, June 2011
04-00124-06-en
Setting an object’s present-value signal to Read/Write (R/W)
allows both monitoring of its value as well as control of it. How-
ever, this means that the Xenta 913 will continuously read the reg-
ister to fetch the latest value even though it is expecting to have
control of it. This is either a waste of network bandwidth because
the value will not be changed externally, or it presents a potential
conflict of control because it can be! In nearly all cases the
Write-only option is preferable because this will cause the Xenta
913 to read the register’s value once at start-up before it assumes
control of it.
• Coefficient Gain and Offset – Allow the raw object’s value to be
converted into the desired absolute units. If the raw register value
is a real number then normally no conversion is necessary and the
default gain and offset of 1 and 0 can be used. But if the raw regis-
ter value is an integer then it often needs to have a gain and offset
applied.
As an example, a power meter might generate voltage values as
unsigned integers with the actual voltage multiplied by 10. In this
case the Xenta 913’s gain should be set to 0.1 to convert the raw
units (10*V) into the required absolute units (V).
• Signal DataType – Is set to a default of BOOL, INTEGER or
REAL based on the selected BACnet object type. But the default
data type may subsequently need to be changed to suit the applied
conversion coefficient. For example, if a gain of 0.1 is applied to
an integer it will produce a real value, so the default DataType
would be changed to REAL in this case.
• Signal Measurement System – The measurement system param-
eters need to be manually set to match the absolute units of the
object’s value after conversion by the coefficient gain and offset,
being either an enumeration or analogue engineering unit as appli-
cable.
Notes
•The W and R/W I/O options should only be selected for output
object types (for example, AO, BO, MO, AV, BV and MV).
• If the W or R/W option is selected then the Write Priority prop-
erty of the interface should be set to resolve any conflict of con-
trol with an external device. If the Xenta 913 is to be guaranteed
control then its Write Priority may need to be increased to
above that of the conflicting control device. Conversely, it the
external device is to be guaranteed control then the Xenta 913
can probably be left with its default Write Priority of 12.