Distech Controls ECB-VAV, User Manual

Page: 17 / 138

BACnet MS/TP Communication Data Bus Fundamentals
Network Guide 15
Device Loading
Each device presents an electrical load on the BACnet MS/TP Data Bus Segment. This is
called device loading. The use of full load devices limits the number of devices connected to a
BACnet MS/TP Data Bus Segment to 32 devices. Distech Controls' BACnet MS/TP devices
are ⅛-load devices and ½-load devices, which allows more devices to be connected to the
BACnet MS/TP Data Bus Segment, as compared to full load devices.
However, if a data bus segment is interoperating with devices that are full-load, ½-load, ¼-
load, or ⅛-load, then the device that supports the fewest devices on the same data bus is the
one that sets the limit for the maximum total number of devices for that data bus segment. For
example, you plan to put on one data bus the following devices:
1. This is limited by the maximum number of master devices allowed on a BACnet MS/TP Data
Bus.
The solution for the above example is to create two data bus segments connected together by
a repeater and then split up the devices between the data bus segments, ensuring again that
the maximum number of devices on each separate data bus is not exceeded. See
Using
Repeaters to Extend the Data Bus on page 28.
Table 3: Device Loading
Manufacturer
Device load on the attached BACnet MS/TP Data
Bus
Distech Controls' ECB controllers
Distech Controls' ECB-PTU Line-Powered
Controllers ⅛-load devices
Distech Controls' BACnet MS/TP Thermostats ½-load devices
Other manufacturers Refer to their documentation
Table 4: Device Loading Example
Manufacturer
Quantity of
devices
(example) Equivalent
full-load
devices
Maximum devices supported by
the manufacturer
Distech Controls' devices
(⅛-load devices) 8 1
128
1
Maximum 50 recommended
Distech Controls' BACnet
MS/TP Thermostats
(½-load devices) 14 7 64
Maximum 50 recommended
Manufacturer Y
(full load devices) 26 26 32
Total Full-Load Devices 34 There are too many devices on the
data bus. It is limited to a maximum
of 32 devices by Manufacturer's Y
devices.