Bacnet Telegrams; Bacnet Ms/Tp Telegram Overview; Telegram Types - Grundfos CIM 300 User Manual

9. BACnet telegrams

9.1 BACnet MS/TP telegram overview

All BACnet MS/TP telegrams have the following format:
Preamble Telegram type
2 bytes:
1 byte
0x55 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
00 Token
01 Poll for master
02 Reply to poll for master
03 Test request
04 Test response
05 BACnet data, expecting reply
06 BACnet data, not expecting reply
07 Reply postponed
Destination Source Length
2 bytes, MSB
1 byte 1 byte
Description
Used to pass network mastership to the destination device.
Discovers the presence of other master devices on the network.
Used by a master to indicate a wish to enter the token ring.
Used to initiate a loopback test.
A reply to a test request telegram.
Used for data transmission where a reply is expected.
Used for data transmission where no reply is expected.
Used by master devices to defer sending a BACnet data reply.
Header CRC Data
Variable,
1 byte
first [0-501] bytes
Data CRC Pad
2 bytes, LSB
At most 1 byte 0xFF
first
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