Civil Engineering Reference
In-Depth Information
RS-CAN hardware performs the sending according to priority rules. This method
has the advantage that the software does not need to check for a free message buffer.
RS-CAN indicates the fill state of the transmit queue.
3.3.3.3
Sending from Multi-Purpose FIFO
A
multi-purpose FIFO
in transmit mode (TX)
has its own memory area to queue
up messages. Again, software writes into a single location to feed in the messages,
and the RS-CAN hardware takes care of sending and fill-level indication. The dif-
ference between this and the
transmit queue
is that within a FIFO, the sequence of
messages will be kept, ignoring any priority rule (message identifier). One transmit
message buffer must be assigned to a FIFO.
3.3.3.4
Sending from GW FIFO
If a
multi-purpose FIFO
is operated in
GW mode
, it can be assigned to be a recep-
tion target for other CAN channels, so that selected messages from them can be
routed to it. In this configuration, the RS-CAN hardware performs all the tasks for
this message routing without any necessary software interaction. Again, one trans-
mit message buffer must be assigned to every multi-purpose FIFO.
The key prerequisites for smooth operation of the GW are that incoming mes-
sages are well selected from the channels by using the appropriate
access filtering
list (AFL)
settings, and that the bus transport capacity of the output CAN channel
is sufficient.
There is only one task that remains for the software, and this is the supervision of
the FIFO overflow. If the configuration of the CAN channels allows (at least tempo-
rarily) more data to be routed through the hardware GW than the output channel can
transmit, the FIFO may run into overflow, so that messages are lost.
3.3.3.5
Transmit History List
Every sent message can be recorded in the
THL
. The THL represents the confirma-
tion for the software that a message has been sent and
acknowledged
by another
CAN bus participant. The THL can generate interrupts on new entries or on fill level.
3.3.3.6
Transmission Intervals
When using the FIFO methods for sending messages, the minimum interval between
two subsequently sent messages can be defined by an internal timer in RS-CAN.
This functionality is required to fulfil
Transport Protocol
requirements of ISO
15765-2 and to avoid a full bus load caused by one node.
