Bacnet Ms/Tp; Bacnet Ms/Tp Message Structure - WEG bacnet CFW701 User Manual

2.1

BACNET MS/TP

In the CFW701, the BACnet protocol was developed using the RS485 standard for the physical and data link
layers, called BACnet MS/TP (Master Slave/Token Passing). BACnet MS/TP nodes can be divided into two
groups, master nodes and slave nodes, according to the node address range.
The access control to the communication mean is performed in two ways:
Master/Slave (MS): It is used in the communication between a master node and a slave node;

Token passing (TP): Communication only among master nodes. A logical ring is defined and the master

that has the token can establish communication with slave nodes and other masters.
In a BACnet MS/TP network, the nodes are initialized and enter the IDLE state, waiting for the arrival of a
telegram, which can be:
Invalid frame: it remains in IDLE;

Not desired frame: it remains in IDLE;

Token: it enters the USE TOKEN state, executes the necessary communication (with slaves or other

masters) and passes the token to the next node;
Reception of a Poll of Master: it sends a telegram to the node with the address in the Source Address field;

Reception of a DataNoReplay: it signalizes the reception to the higher layers;

Reception of a DataNeedingReplay: it signalizes the reception to the higher layers and sends the requested

response.
2.1.1

BACnet MS/TP Message Structure

The BACnet specification defines that the frame can have from 0 up to 501 bytes (octets), and that each byte is
composed by 8 bits without parity, with start and stop bits, as showed in the Figure 2.2.
Reception (RX): The maximum time between bytes (Tframegap) is of 20-bit times, and the minimum time
between frames (Tturnaround) after the last byte stop bit is of 40-bit times, according to the Figure 2.3.
Transmission (TX): The RTS signal must be disabled after the end of the stop bit time (Tpostdrive), which is of
15-bit times.
Header and data, as Figure 2.4 illustrates, form the BACnet data frame.
Introduction to the BACnet Communication
Figure 2.2: Byte structure
Figure 2.3: BACnet data reception
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