Bacnet Telegrams; Bacnet Ms/Tp Telegram Overview; Telegram Types - Grundfos CIU 300 Functional Profile And User Manual

10. BACnet telegrams

10.1 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. See section
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.
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
22
Destination Source
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.
Length Header CRC
2 bytes,
1 byte
MSB first
3.7 Selecting the BACnet MAC address
Data Data CRC
Variable, At most
2 bytes,
[0-501]
LSB first
bytes
(Pad)
1 byte
0xFF
.