Viconics VT8600 Series BACnet Integration Guide - Manual (Page 28)

VT8600 Series BACnet ® Integration Manual 28
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Free Programmed Object or Loops
Read and write MS/TP data on an “If Once” basis or a “Do Every Loop" basis instead of reading or writing
to a third-party device’s object directly in the program. Otherwise, any read or write request occurs at the
Supervisory Controller’s program scan rate, which may be in hundredths of milliseconds. This can bog down
a network as single commands can be sent to all ASC devices down the MS/TP trunks every hundredths of
milliseconds
Programs writing to the devices should have a structure similar to the following:
If Once Schedule = On then
MV10 = Occupied
End If
If Once Schedule = Off Then
MV10 = Unoccupied
End If
OR
Do Every 5min
If Schedule = On Then
MV10= Occupied
Else
MV10 = Unoccupied
End If
End Do
Retries and Timeouts
In BACnet ® integration, you should note the device object of the Supervisory Controller and Operator’s
Workstation. This object contains the two following required properties:
1) Retry Timeout
2) Number of APDU Retries
The Retry Timeout specifies the time between re-transmissions if the acknowledgement is not received.
Increasing this value may help if you are experiencing problems with controllers dropping off line.
The Number of APDU Retries specifies the number of times unsuccessful transmissions are repeated. If the
receiving controller has not received the transmission successfully after this many attempts, no further attempts
will be made.
For example, if one of the controllers does not reply to a Supervisory Controller (SC) request, and the SC’s Retry
Timeout is set to 2000 milliseconds and the Number of APDU Retries is set to 1 (SC level), the SC sends one
other request 2 seconds later. If the MS/TP device does not reply, it is considered off line by the workstation.
Having a Retry Timeout value of 10450 milliseconds and a Number of APDU Retries property set to 3 at the SC
level may prevent the device from dropping off line. These properties should also be changed at the workstation
level since the workstation issues requests to any MS/TP devices when the graphics are used.