Using the cpx8xx usb bus coupler – BECKHOFF CP-Link3 User Manual
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Page 60
Using the CPx8xx USB Bus Coupler
Bus Couplers of type CPx8xx are integrated into Beckhoff Panel PCs. Various different I/O terminals are connected
through these Bus Couplers. In addition to the standard I/O channels of a Panel PC, further I/O terminals can be
connected through which, for instance, special keys, LEDs, rotary switches or similar items can be operated. When a
Panel PC is used as a CP-Link 3 client there are different possible methods for accessing the client's integrated USB
Bus Coupler from the host PC:
1. Using via the CP-Link 3 Virtual USB hub
2. Access via "Virtual USB interface (Remote)"
a. via standard Ethernet
b. via RT-Ethernet
3. Access via network variables (RT Ethernet)
4. Access via ADS to task or PLC variables linked locally on the client
a. via standard Ethernet
b. via RT-Ethernet
The difference between the variants is explained in the following table:
Bus Coupler logically
connected with
I/O access from
RT
Ethernet
Configuration
effort
Dropout of outputs possible
(see explanation underneath
the table)
Host
PC
Client PC
Host
PC
Client
PC
1
X
X
low
Yes
2.a
X
X
low-medium
Yes
2.b
X
X
X
medium
No
3
X
X
X
medium-high
No
4.a
X
X
high
No
4.b
X
X
X
high
No
Possible dropout of the USB Bus Coupler outputs:
The USB Bus Coupler built-in to the client expects a certain time interval, in which it receives data to write to its outputs.
In the standard configuration of a CPx8xxx Bus Coupler this time interval is 100 ms. If this time interval cannot be met,
the outputs of the Bus Coupler drop out. Depending on the usage type of the USB Bus Coupler, in practice under certain
conditions, the time interval may not be met and the outputs of the Bus Coupler drop out. Generally, this seldom if ever
happens, but cannot be ruled out altogether. Reasons for this are the type of network connection of the Bus Coupler
integrated into the client to the host PC or the operating system architecture of the host PC. If the I/O access of the host
PC takes place via a standard Ethernet connection, during data exchange with the Bus Coupler, a lot of small data
packets per cycle must be sent consecutively over the IP connection. Depending on the network load and the client CPU
load sending of these packets can take longer than the time interval defined.
To be certain that the outputs do not drop out under any circumstances, I/O access from the host PC has to be done via
RT Ethernet, or the I/O access must take place on the client PC itself. If I/O access takes place on the client itself this
does however increase the configuration effort as the inputs and outputs of the Bus Coupler cannot be used directly on
the host PC, but must be made available via ADS or network variables.
BECKHOFF CP-Link 3: Miscellaneous