Beckhoff EL6751 Documentation page 106

Parameterization and commissioning
Cyclic Synchronous
In transmission types 1-240 the PDO is transmitted cyclically: after every "nth" SYNC (n = 1...240). Since
transmission types can be combined on a device as well as in the network, it is possible, for example, for a
fast cycle to be agreed for digital inputs (n = 1), whereas the data for analog inputs is transmitted in a slower
cycle (e.g. n = 10). RxPDOs do not generally distinguish between transmission types 0...240: a PDO that has
been received is set to valid when the next SYNC is received. The cycle time (SYNC rate) can be monitored
(object 0x1006), so that if the SYNC fails the device reacts in accordance with the definition in the device
profile, and switches, for sample, its outputs into the error state.
The FC510x card / EL6751 terminal fully support the synchronous communication method: transmitting the
SYNC telegram is coupled to the linked task, so that new input data is available every time the task begins. If
a synchronous PDO does not arrive, this is detected and reported 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
transmitted 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 automatically (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 / terminals.
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 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.
Fig. 118: Timing diagram: "Inhibit time"
Although the Beckhoff FC510x PC cards / EL6751 terminal 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.
106 Version: 3.5 EL6751