I/O
Application Note DK9222-1109-0015
Wireless Automation
Industrial wireless at the field level
Keywords
Wireless
Radio
WLAN
Coexistence
Antenna
KM6551
CU8890
Tips for the commissioning of industrial wireless networks are described in this application example.
Through the use of radio technology in automation technology, the installation effort and the costs can
be minimised in accordance with the plant performance. The first part describes all of the necessities for
an error-free radio link, while typical installation errors and their rectification are described in the second
part.
1. Wireless I/O-integrated: KM6551 data exchange terminal and CU8890 WLAN controller
Wireless solutions are the first choice in rough terrain and also in the case of moving components, in order to avoid expensive
installations and intensive maintenance costs and at the same time to achieve ideal plant performance. Beckhoff offers I/
O‑integrated products here on the basis of standardised basic technology:
The KM6551 data exchange terminal for the Beckhoff Bus Terminal system uses the IEEE 802.15.4 with 16 independent
channels in combination with the robust DSSS method. DSSS stands for Direct Sequence Spread Spectrum and, as a frequency
spreading method, offers good physical conditions for wireless data transmission. A proprietary, fast protocol based on
IEEE 802.15.4 (2.4 GHz band) and DSSS and adapted for automation technology was developed for the KM6551, so that
communication is only possible between KM6551 terminals, but the worldwide use of this technology is assured. Status and
data exchange are displayed directly by LEDs and offer fast and simple diagnostics. A library is available for the use of the
KM6551 module with TwinCAT. The industrial CU8890 WLAN controller supports WLAN based on the IEEE 802.11 b/g standard
and is designed for control cabinet installation. When connected to a Beckhoff Industrial PC, the module can be used both as
an access point and as a client and thus enables access to available infrastructures. A USB port is required on the Industrial PC
for connection. Eleven channels in the 2.4 GHz band are available for selection. Status and data transmission are displayed by
LEDs and offer fast and simple diagnostics. A comprehensive range of cable and antenna accessories is available for both the
KM6551 and the CU8890.
For application notes see disclaimer on the last page
Beckhoff
New Automation Technology
1
Application Note DK9222-1109-0015
Wireless Automation
I/O
The 2.4 GHz band is the only suitable, licence‑free, worldwide usable frequency band for industrial automation, as the relatively
large bandwidth of 80 MHz has acceptable propagation characteristics. Wireless solutions that rely on this technology are very
well suited to industrial automation and deliver good results in practice. However, all of that is of little use if one is not familiar
with the basic principles of radio technology and when troubleshooting during commissioning takes a long time. Automation
processes require rapid deterministic communication, which is dependent on both the frequency band and the topology in
radio‑based data transmission.
For application notes see disclaimer on the last page
Beckhoff
New Automation Technology
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I/O
Master
Slave
send output data
send input data
Option 1 | Data exchange peer-to-peer
Master
Slave 1 Slave 2 Slave 3 Slave 4 Slave 5 Slave 6 Slave 7
Option 2 | Data exchange up to max. 7 devices
Master
Slave 1 Slave 2 Slave 3 Slave 4 Slave 5
...
Slave x
Option 3 | Broadcast up to x devices
Application Note DK9222-1109-0015
Wireless Automation
KM6551 – operating modes: PTP, star, broadcast
With the three operating modes of the KM6551 data exchange terminal, 10 bytes of process data can be exchanged
independently of the higher level fieldbus or transmitted via radio between two independent controllers. With the KM6551
Bus Terminal Beckhoff offers a cost‑effective, high‑performance option for establishing PTP, star and broadcast configurations
via radio.
Fig. 1 KM6551: operating modes PTP (left), star (centre) and broadcast (right)
In PTP mode (peer‑to‑peer), two independent controllers communicate bidirectionally with each other and exchange 10‑byte
input and output data cyclically. The complete exchange takes 20 ms. The free‑field distance between two KM6551 units can be
up to 300 m in PTP mode.
In star mode, the master communicates unidirectionally with up to seven slaves; the 10 bytes of process data are distributed
cyclically to all devices and the exchange takes 20 ms per slave. The slaves are directly addressed by the master, so that the
temporal sequence of the communication is programmed from the PLC.
In broadcast mode, the master sends new data to all broadcast slaves unidirectionally in a 10 ms cycle. The slaves receive
these data, but cannot send any data themselves. Any number of slaves can listen in.
For application notes see disclaimer on the last page
Beckhoff
New Automation Technology
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I/O
Laptop
USB
CU8890
USB
1
USB
2
USB
n
CU8890 CU8890 CU8890
CU8890 as access point CU8890 as client
Access point (corporate network)
Application Note DK9222-1109-0015
Wireless Automation
CU8890 – operating modes: client or access point
Client drivers are available for Windows XP, Embedded Standard and Windows CE, and therefore for any Beckhoff Industrial
and Embedded PC. Due to the drivers for Windows XP and Embedded Standard, the CU8890 can also be operated as an
access point. Encryption mechanisms are possible with AES 128‑bit up to WPA2. The module is Cisco‑CCX‑compatible and
supports PEAP and LEAP. The data rate is adjusted dynamically up to 54 Mbit/s (gross). The CU8890 has a reverse SMA plug for
connecting the antenna, this way the antennas can be adapted to the environment.
Fig. 2 left – access point, right – WLAN client
2. Coexistence – simultaneous operation of radio systems
A radio network is a jointly used medium; therefore, all participants must share the transmission link. It is ensured within
the network that the available bandwidth is distributed among the devices. There are two possible methods here. Either one
device may use the entire bandwidth for a certain period of time while all other devices wait. Upon expiry of this transmission
time, the communication is interrupted and it is then the next device’s turn. Or a device is provided only with one small
part of the bandwidth (a channel), which it can use for data transmission. The following systems use the 2.4 GHz band:
WLAN, IEEE 802.15.4, Bluetooth and other proprietary technologies. Proprietary technologies are systems made by various
manufacturers that are not standardised, e.g. baby phones, radio transmission systems for video signals and the like.
For application notes see disclaimer on the last page
Beckhoff
New Automation Technology
4
Application Note DK9222-1109-0015
2412 2417 2422 2427 2432 2437 2442 2447 2452 2457
2462
2467 2472
1 2 3 4 5 6 7 8 9
10
11
2402 2407
Channel:
MHz
2400 2483,5
2405 2410 2415 2420 2425 2430 2435 2440 2445 2450 2455 2460 2465 2470 2475 2480
1514131211109876543210
2400 MHz
2412 MHz 2437 MHz 2462 MHz
22 MHz
CU8890
(WLAN)
KM6551
(IEEE 802.15.4)
2
Frequency in MHz
Channels
Channel 1 Channel 6 Channel 11
Wireless Automation
Fig. 3 Overlapping of WLAN channels – only 1, 6 and 11 are possible in parallel
If radio systems with the same frequency are operated at the same time and in the same place, interference can occur. In
I/O
order to prevent this, most systems have several separate transmission channels (e.g. WLAN and IEEE 802.15.4), so that the
user can operate the systems in parallel. The eleven WLAN radio channels are arranged in an overlapping manner. Therefore,
only channels 1, 6 and 11 can be operated independently in parallel (see fig. 3). Corresponding free frequency ranges are then
available for the channels of the KM6551, so that both can be operated in parallel. In the following example (fig. 4), CU8890
WLAN Controllers with eleven channels transmit in the same range as KM6551 units with 16 channels. The channels can be
freely set in both the CU8890 and the KM6551, allowing several systems to be operated simultaneously.
Fig. 4 Parallel operation of WLAN (CU8890) and IEEE 802.15.4 (KM6551)
For application notes see disclaimer on the last page
Beckhoff
New Automation Technology
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