wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

42-4 Industrial Communication Systems
Length
Octet 0 Octet 1 Octet 2 Octet 3 Octet 4 Octet 5 Octet 6 ...
Control
Source address
Destination address
User data (payload)
Octet 22 (max)
Check octet
Frame type
Repeated
Priority
Sender:
Individual
address
Receiver(s):
Individual
or
group
or
broadcast
address
Address type
Hop count
Length
TPDU
FCS
TP1/PL110 standard data frame
Length
Octet 0 Octet 1 Octet 2 Octet 3 Octet 4 Octet 5
Control Ext. control Source address
Destination address
Octet 6
Octet 7
...
User data (payload)
Check octet
Frame type
Repeated
Priority
Address type
Hop count
Extended
frame
format
Sender:
Individual
address
Receiver(s):
Individul
or
group
or
broadcast
address
Length
TPDU
FCS
TP1/PL110 extended data frame
...
TL
control
field
AL
control field
TPDU
AL
user data
FIGURE 42.2
Frame formats.
two octets specify the source address. Regardless of the used communication service, this is always
the individual address of the sending device. The succeeding two octets identify the destination of
the message. The address type (AT) bit indicates whether the destination is an individual address or
a group address. Together with the hop count used by the NL and the frame length, the AT bit makes
up the last octet before the NL payload. The value of the Length field is the number of NL payload
octets minus one. The frame ends with a check octet (FCS, frame checking sequence), which holds the
vertical odd parity of all preceding octets (each Bit is calculated as the logical XOR over all Bits of the
same significance, then inverted).
The format of the TL protocol data unit (TPDU), that is, the NL payload, is shown at the bottom of
Figure 42.2. On KNX, it is the lowest layer PDU whose format is independent of the communication
medium. The width and alignment of the AL protocol control information and user data differ depending
on the TL and AL service type as well as the size of the AL payload.
The standard data frame supports TPDUs with a maximum length of 16 octets. This corresponds to
a maximum AL payload of 14 octets. For the transmission of longer TPDUs, the extended data frame
format has been defined. It is shown in the middle of Figure 42.2. Extended frames have an additional
control field (Extended Control) as their second octet. Besides the AT Bit and the hop count, this octet
contains a 4 bit field that holds additional format information (Extended Frame Format). The length
field is expanded from 4 to 8 bit.
Standard and extended data frames are usually followed by a layer 2 acknowledgment frame sent by
the receiver, which is 1 octet long. If group addressing is used on the twisted-pair medium, all receivers
send this frame at the same time; negative acknowledgments will cancel out positive ones. The
L_Poll_Data frame format (which is rarely used) is roughly similar to L_Data, with the master
transmitting a request consisting of the control and address part followed by a check octet and the slaves
responding with one octet of user data each.
On the powerline medium, the formats of standard and extended data frames are identical to those
defined for twisted-pair segments. However, a preamble and receiver training sequence are added in
© 2011 by Taylor and Francis Group, LLC