BECKHOFF FC5101 - Manual (Page 46)

Eiserstraße 5 / D-33415 Verl / Telefon 05246/963-0 / Telefax 05246/963-149
46
switches, for example, its outputs into the fault state.
The FC510x PC cards support cyclic synchronous transmission types completely: transmitting the SYNC tele-
gram is coupled to the linked task, so that new input data is available every time the task begins. The card will
recognise the absence of a synchronous PDO, and will report it to the application.
Only RTR
Transmission types 252 and 253 apply to process data objects that are transmitted exclusively on request by a
remote frame. 252 is synchronous: when the SYNC is received the process data is acquired. It is only transmit-
ted on request. 253 is asynchronous. The data here is acquired continuously, and transmitted on request. This
type of transmission is not generally recommended, because fetching input data from some CAN controllers is
only partially supported. Because, furthermore, the CAN controllers sometimes answer remote frames auto-
matically (without first requesting up-to-date input data), there are circumstances in which it is questionable
whether the polled data is up-to-date. Transmission types 252 and 253 are for this reason not supported by the
Beckhoff PC cards.
Asynchronous
The transmission types 254 + 255 are asynchronous, but may also be event-driven. In transmission type 254,
the event is specific to the manufacturer, whereas for type 255 it is defined in the device profile. In the simplest
case, the event is the change of an input value - this means that every change in the value is transmitted.The
asynchronous transmission type can be coupled with the event timer, thus also providing input data when no
event has just occurred.
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 trans-
mission 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 parameterise the inhibit time on slave devices, they do not them-
selves 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 subindex 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.