Beckhoff CX8050, Документация

Страница: 76 / 169

CAN
CX8050, CX8051 - Embedded-PCs for
CANopen and CAN
76 Version: 1.4
Inhibit time
The ”inhibit time" parameter can be used to implement a ”transmit filter" that does not increase the reaction
time for relatively new input alterations, but is active for changes that follow immediately afterwards. The
inhibit time (transmit delay time) specifies the minimum length of time that must be allowed to elapse
between the transmission of two of the same telegrams. If the inhibit time is used, the maximum bus loading
can be determined, so that the worst case latency can then be found.
Although the BECKHOFF FC510x PC cards can parameterize the inhibit time on slave devices, they do not
themselves support it. The transmitted PDOs become automatically spread out (transmit delay) as a result of
the selected PLC cycle time - and there is little value in having the PLC run faster than the bus bandwidth
permits. The bus loading, furthermore, can be significantly affected by the synchronous communication.
Event Timer
An event timer for transmit PDOs can be specified by sub-index 5 in the communication parameters. Expiry
of this timer is treated as an additional event for the corresponding PDO, so that the PDO will then be
transmitted. If the application event occurs during a timer period, it will also be transmitted, and the timer is
reset.
In the case of receive PDOs, the timer is used to set a watchdog interval for the PDO: the application is
informed if no corresponding PDO has been received within the set period. The FC510x can in this way
monitor each individual PDO.
Notes on PDO Parameterization [
}
78]
PDO Mapping
PDO mapping refers to mapping of the application objects (real time data) from the object directory to the
process data objects. The CANopen device profile provide a default mapping for every device type, and this
is appropriate for most applications. Thus the default mapping for digital I/O simply represents the inputs and
outputs in their physical sequence in the transmit and receive process data objects.