9. BACnet telegrams
9.1 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 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
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.
Related information
2.7 Selecting the BACnet MAC address
9.2 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 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.
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