Civil Engineering Reference
In-Depth Information
Fig. 4.8
PDO protocol
are available for the, to be transmitted, process data (see Fig.
4.8
). Due to this the
PDO does not need an overhead, in contrast to the CAN protocol, and therefore no
additional bus bandwidth is necessary. The PDOs are configured with the help of
two parameter sets: PDO communication parameter and PDO mapping parameter.
The communication object identifier (COB ID), the transmission type, the inhibit
time, the event timer and the SYNC start value belong to the PDO communication
parameters. The 32-bit parameter COB ID consists of besides a few control bits,
similar to the corresponding SDO parameter (see Fig.
4.7
), the CAN identifier used
for transmitting or receiving. The parameter transmission type defines how the PDO
is to be transmitted or sent. The transmit PDO (TPDO) differs between a cyclic syn-
chronous and an event-driven (asynchronous) transmission. When using the event-
driven transmission, the event has to be defined in the device or application profile
(transmission type: 255) or by the manufacturer (transmission type: 254). The event
could be a temperature increase of 1°C or the change of a binary signal. The event
could also be the expiration of the event timer. In this case, the PDO is sent periodi-
cally with the time (in milliseconds) configured in the event time parameter. If a
signal-specific event and the event timer are defined, the PDO will be transmitted
directly after the signal-specific event occurs. If no signal-specific event occurs, the
PDO will be transmitted after the event timer has been elapsed. The event timer is
restarted after every PDO transmission.
When a PDO gets transmitted cyclic synchronously (transmission type: 1), the
process data are updated and transmitted after receiving a SYNC message. In order
not to unnecessarily increase the busload with low-frequency signals, the PDO can
be sent with only every second or 240th SYNC message (transmission type: 2-240).
With the parameter SYNC start value, it is possible to configure the counter-value
of the SYNC message (
sync counter
) to determine the first transmission. A pos-
sibility of reducing the busload lies in the usage of an acyclic synchronous trans-
mission (transmission type: 0) in which the SYNC message has to be received and
additionally a defined event has to occur. For receive-PDOs (RPDOs), a differentia-
tion is only made with asynchronous or synchronous reception. With asynchronous
reception the process data are immediately handled, whereas with synchronous re-
ception the device waits for the next SYNC message to actuate the received data.
