BACnet CFW-11, User Manual

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Introduction to the BACnet Communication
CFW-11 | 11
HEADER DATA
0x55 0xFF
Frame
type
Destination
address
Source
address
Length Length CRC Data CRC CRC
Figure 3.4: BACnet Frame
Preamble: It is formed by two bytes with the 55h and FFh values respectively.
Frame type: The BACnet specification defines eight frame types, from 0 to 7. Frame Types 8 through 127 are
reserved for the specification improvement, and from 128 through 255 are reserved for each vendor specific
frames. The defined types are:

0 Token

1 Poll for master

2 Reply to poll for master

3 Test request

4 Test response

5 BACnet data expecting reply

6 BACnet data not expecting reply

7 Reply postponed
Only master nodes must acknowledge frame types 0, 1 and 2, the slave nodes must ignore them.
Token (0) frame type: it is used in the relationship between master nodes. It does not present data. The master
node that has the token can initiate the communication. After sending the maximum number of data frames
(Nmax_info_frames) and waiting any expected replies, it must pass the token to the next master.
Poll for Master (1) frame type: It is transmitted periodically during the configuration. It is used to discover the
presence of other masters in the network and to determine the token sequence. Master nodes must respond
and slave nodes must ignore it. It does not present data.
Reply to Poll for Master (2) frame type: It is the response of the master nodes to the Poll for Master (frame type
1). It does not present data.
Test Request (3) frame type: It is used to start the communication in the MS/TP network. It is applied to send a
particular piece of information to a node.
Test Response (4) frame type: It is the response to a Test Request.
BACnet Data Expecting Reply (5) frame type: It is used by master nodes to convey the data parameter of a
DL_UNITDATA.request that presents destination address, data, priority and message code, waiting for a
response from the destination node.
BACnet Data not Expecting Reply (6) frame type: It is used by master nodes to convey the data parameter of a
DL_UNITDATA.request that presents destination address, data, priority and message code. It does not wait for
a response from the destination node.
Reply Postponed (7) frame type: It is used by master nodes to indicate that the response to a Data Expecting
Reply frame will be sent later. It does not present data.
Destination and source addresses: It is formed by two bytes, destination and source, respectively.
Length: It is formed by two bytes that inform the number of data bytes in the message.
Header CRC: The last part of the header is the fields for checking header transmission errors. The used
method is the CRC-8 (Cycling Redundancy Check).
Data: It may present from 0 to 501 bytes, according to the BACnet specification. Data in the CFW-11 can
present up to 59 bytes.
Data CRC: The last part of the telegram is the field for checking data transmission errors. The used method is
the CRC-16 (Cycling Redundancy Check).