Distech ECYPSE - Device Loading

BACnet MS/TP Communication Data Bus Fundamentals
ECLYPSE User Guide 113
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.
Table 11-3: Device Loading
Manufacturer
Device load on the attached BACnet MS/TP Data Bus
Distech Controls’ ECB and
ECY Series controllers
Distech Controls’ ECB-PTU
Series Line-Powered
Controllers
⅛-load devices
Distech Controls’ BACnet
MS/TP Thermostats
½-load devices
Other manufacturers
Refer to their documentation
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:
Table 11-4: 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.
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 122.
Baud Rate
Most devices will have a range of baud rate settings and possibly an AUTO setting that
detects the baud rate of other devices transmitting on the data bus and adjusts the baud rate
of the device accordingly. Typical baud rates are 9600, 19 200, 38 400, and 76 800. The
baud rate setting determines the rate at which data is sent on the BACnet MS/TP data bus.