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

KNX 42-5
front, and the domain address follows as a postamble. Acknowledgment frames can only be sent by
a single group member. The total length of extended frames (which are practically never used on this
medium) is limited to 73 octets.
The KNX radio frequency (RF) frame format has supported TPDUs longer than 16 octets from its
inception. Therefore, the difference between standard and extended data frames does not exist on this
medium, and a slightly different format is used. The cEMI frame format that is used for tunneling KNX
data over IP networks is similar to the extended data frame format as described.
In addition to network protocol messages, the KNX standard defines the use of so-called external
messages which are specified in the external message interface (EMI). EMI specifies a standard protocol
for accessing services of the KNX network stack and management server entities. Its messages can be
sent via various lower layer protocols, which are also part of the KNX specification. EMI is mainly used
for local point-to-point connections where external user applications communicate with a connected
bus attachment unit (BAU). A typical example would be a management client like a workstation with
a USB or EIA-232 connection to a device that provides a connection to the KNX bus medium. Various
EMI versions that differ in the available service types and in the used encodings exist: EMI1, EMI2, and
cEMI (common EMI). In contrast to other EMI versions, cEMI is also used for KNXnet/IP and thus the
only version to be used outside the context of local point-to-point connections.
42.3 Medium-Dependent Layers
The current release 2.0 of the KNX Handbook specifies three different physical layers for communication.
Communication over IP networks as a first class medium is included as a draft specification. The
resulting four options are described in the following. All media can be deployed and combined using
routers according to an integrator’s need.
Twisted Pair 1 (TP1) is the oldest and still most popular medium for KNX. It was taken from EIB.
A single-wire pair (e.g., ½ YCYM 2 × 2 × 0.8) is used for both data and power transmission. A TP1 segment
can accommodate up to 256 devices in free topology. It may extend up to an accumulated cable length of
1000.m and must contain at least one bus power supply unit to provide the link power of about 30.V DC.
The contention protocol being used is carrier-sense multiple access (CSMA) with collision avoidance by
bit-wise arbitration. The dominant logical 0 is encoded as a negative voltage imprinted on the DC supply
voltage, whereas a logical 1 corresponds to the idle state of the medium. Data are transferred character
oriented via half-duplex bidirectional communication at a transmission rate of 9600.bit/s. Characters contain
eight data bits, even parity, and one stop bit (8e1) and are separated by a minimum idle period of 2 bit
times. Data frames of variable length may be transmitted after a line idle time of 50 bit times. An immediate
acknowledgment frame may follow after a gap of 15 bit times. TP1 is the only medium to support polling
groups. Domain addresses are not used on TP1. The Batibus twisted-pair medium was also part of the
KNX standard under the name of TP0, but was removed from release 2.0.
The design of the KNX RF physical layer and DL is compatible with Wireless M-Bus, a European metering
protocol standard, allowing tight integration. KNX RF operates at a center frequency of 868.3.MHz,
which is located within the band reserved for SRDs (Short Range Devices). In the 868.0–868.6.MHz subband,
a duty cycle limitation to less than 1% applies. Data are transmitted at a data rate of 16.4.kbps using
frequency shift keying (FSK) modulation and Manchester encoding. The KNX RF frame format is based
on FT3 as specified in IEC 60870-5. It provides the capabilities of the extended frame format as described in
the previous section and contains additional medium-related information such as signal quality or battery
state. Retransmitters can be used to extend the range of the network. To allow detection and elimination
of duplicate frames that can occur in particular when such retransmitters are employed, the TP1/PL110
repeat flag is replaced by a 3 bit link layer frame number (LFN). KNX RF allows the use of unidirectional,
transmit-only devices. Such devices are cheaper to produce and have a greatly extended battery lifetime
due to reduced power consumption, but accommodating them requires an adapted addressing scheme.
Therefore, an extended address format composed of the unique serial number of the device concatenated
© 2011 by Taylor and Francis Group, LLC