11. BACnet telegrams
11.1 BACnet MS/TP telegram overview
All BACnet MS/TP telegrams have the following format:
Preamble
Telegram
type
Destination
Source
Length
Header CRC
Data
Data CRC
Pa
2 bytes:
0x55 0xFF
1 byte
1 byte
1 byte
2 bytes, MSB
first
1 byte
Variable
[0-501] bytes
2 bytes, LSB
first
At most 1 byte
0xFF
For BACnet MS/TP, the destination address and source address are MAC addresses. A destination address of 255 (0xFF) denotes broadcast.
The length field specifies the length in bytes of the data field which must be between 0 and 501 bytes long.
11.1.1 Telegram types
The available telegram types are listed below:
Type
Name
Description
00
Token
Used to pass network mastership to the destination device.
01
Poll for master
Used to discover the presence of other master devices on the network.
02
Reply to poll for master
Used by a master to indicate a wish to enter the token ring.
03
Test request
Used to initiate a loop-back test.
04
Test response
A reply to a test request telegram.
05
BACnet data, expecting reply
Used for data transmission where a reply is expected.
06
BACnet data, not expecting reply
Used for data transmission where no reply is expected.
07
Reply postponed
Used by master devices to defer sending a BACnet data reply.
12. Supported services
BACnet Interoperability Building Blocks (BIBBs) are collections of one or more BACnet services. These are described in terms of an "A" and a
"B" device. Both devices are nodes on a BACnet inter-network. In most cases, the "A" device will act as the user of data (client), and the "B"
device will be the provider of this data (server).
The MIXIT unit is a BACnet Application-Specific Controller (BASC) with a few additional services.
12.1 Data-sharing services
Name
BACnet BIBB code
Description
Initiate
Execute
ReadProperty
DS-RP-B
The MIXIT unit can be the provider of data.
-
●
ReadPropertyMultiple
DS-RPM-B
The MIXIT unit can be the provider of data and return
multiple values at one time.
-
●
WriteProperty
DS-WP-B
The MIXIT unit allows a value to be changed over the
network.
-
●
WritePropertyMultiple
DS-WPM-B
The MIXIT unit allows multiple values to be changed over
the network.
-
●
SubscribeCOV
DS-COV-B
The MIXIT unit can be the provider of "Change Of Value"
data. The unit supports up to 10 simultaneous COV
subscriptions. Subscription lifetime can be limited or
unlimited.
-
●
ConfirmedCOVNotification
●
-
UnconfirmedCOVNotification
●
-
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