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10. BACnet telegrams
10.1 BACnet MS/TP telegram overview
All BACnet MS/TP telegrams have the following format:
For BACnet MS/TP, the destination address and source address are MAC addresses. See section
3.7 Selecting the BACnet MAC address
.
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.
10.2 Telegram types
The available telegram types are listed below.
Preamble
Telegram type
Destination
Source
Length
Header CRC
Data
Data CRC
(Pad)
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
Type
Name
Description
00
Token
Used to pass network mastership to the destination device.
01
Poll for master
Discovers 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 loopback 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.
