SKG Steam Humidifier
BACnet® Communication Module User Guide
21
Configuration
Configuring in the Configuration Mode
If the NEPICSKG is put into the configuration mode while it is “hot”, the NEPICSKG will be automatically reset
with the Device_Instance, Object_Name, Max_Master and
MSTP_MACaddress
all containing the
configuration
mode values
. In this mode, the NEPICSKG will not act as an MS/TP token-passer and will be silent until it is
addressed by a BACnet client. As a side effect, it will also not be able to respond to BACnet WhoIs services
with Iam services, so BACnet clients will not be able to find out its Device_Instance automatically. In the
configuration mode, any of the above properties can be changed by using either the default Device_Instance or
the wild card Device_Instance (4194303 decimal or 0x3FFFFF hex). Use of the wild card Device_Instance
obviates the need to know the NEPICSKG’s real Device_Instance in case it conflicts with another Device in the
system. While in the configuration mode, only the Device Object is available to BACnet clients through the
ReadProperty and Write Property services. All other objects (i.e. AI’s, etc) are not available.
As an alternative to using the BACnet WriteProperty service to change the Device_Instance and/or the
MSTP_MACaddress
, the SK300’s keypad can be used to change those two properties, but only while in the
configuration mode.
Changes to the Device Object, whether made by using the WriteProperty of the keypad, do not take effect until
the NEPICSKG is restarted in the operational mode.
Selecting the MS/TP MAC address and Max_Master
Some care must be taken in setting the MS/TP MAC address and Max_Master property.
First, the MAC address must be unique on the entire MS/TP network. However, having a unique MAC address
and a high baud rate does not guarantee efficient operation of the NEPICSKG (or other MS/TP units for that
matter) on the MS/TP network. Some MAC address and Max_Master combinations are more efficient than
others. BACnet requires token-passing units to occasionally “poll” for other masters based on the value of MAC
address and Max_Master. So, a “poor” combination of MAC addresses and Max_Masters can lead to slow
networks in which there’s a lot of wasted time polling for masters that are not present and never will be. In fact,
unless there are 126 other units on the MS/TP network, the default Max-Master=127 is most likely a poor choice
for the NEPICSKG. Having said that, Max-Master=127 has been chosen as the default to insure that any
master, specifically a BACnet client, can be found when the NEPICSKG is first started.
So, considering the following simple two-unit examples:
Example 1:
•
MAC=0.
Max_Master=127
•
MAC=1,
Max_Master=127
This example is slow and inefficient because both Max_Master=127. Everytime either unit is required to find
another master units it has to poll 126 units until it finds the right one to pass the token to.
Example 2:
•
MAC=0.
Max_Master=5
•
MAC=5,
Max_Master=5
This example is better but is still slower than it could be. The Max_Master is set to the most efficient value,
however because of the gap between the two MAC addresses, each unit must poll 4 units until it finds the right
one to pass the token to.
Example 3:
•
MAC=0.
Max_Master=1
•
MAC=2,
Max_Master=2
This example is actually an incorrect configuration, in that MAC=0 will never find MAC=2 because it will never
poll for the master MAC address=2.
