Commissioning and configuration
64
MA 235
i
Leuze electronic
12.4.5
PDOs
PDOs are objects (data, variables and parameters) from the object directory compiled
(mapped) by the device manufacturer. A maximum of 8 bytes of user data from various
objects can be mapped into one PDO.
A PDO can be received and evaluated by each participant (node). The model is referred to
as the producer-consumer procedure.
Since there is no protocol structure in the telegram of a PDO, the participants in the network
for whom these data are intended must know how the user data in the data area of the PDO
are structured (which data are stored where in the user data area).
The exchange of process data is supported by the MA 235
i
via the following accesses:
• Event-controlled data transfer
Here, the data of a node are transmitted as a message whenever a change to the pres-
ent state occurs.
• Polling with remote frames
The CAN node which has been defined as master in the network requests the desired
information via query (via remote frame). The participant which has this information
(or the required data) responds by sending the requested data.
• Synchronized mode
CANopen permits simultaneous querying of inputs and states of different participants
and the simultaneous change of outputs or states. For this purpose, one uses the syn-
chronization telegram (SYNC) transmitted by a master.
The SYNC telegram is a broadcast to all network devices with high priority and without
data content. Generally, the master sends the SYNC telegram cyclically.
Participants working in synchronized mode read their data when receiving the SYNC
message and then transmits them immediately afterwards as soon as the bus permits
this (see explanation regarding arbitration process).
As the SYNC process can very quickly lead to high bus loads, another distinction is
made between "event-controlled synchronization" and a "timer synchronization".
• Time-controlled transmission
In this case, the transmission of a PDO is triggered when an adjustable time period
has elapsed. The time-controlled transmissions are set individually for each PDO via
the so-called "inhibit time" or an "event timer". The respective parameters can be
found in the objects 1800
h
to 1803
h
for the corresponding PDOs.
• Node monitoring
Heartbeat and guarding mechanisms are available for failure monitoring of the
MA 235
i
. This is particularly important for CANopen, as the MA 235
i
may not respond
regularly in the event-controlled operating mode. In case of guarding, the participants
are cyclically queried for their state via data request telegrams (remote frame). In case
of heartbeat, the nodes transmit their state themselves.
Heartbeat and guarding / life time are standard communication objects from the
DS301 CANopen specification. The corresponding objects here are:
- Heartbeat 1017
h
- Guarding / Life time factor 100C
h
and 100D
h
