SENT: Gateway from SENT to the CAN-Bus 405
imc CANSAS Users Manual - Doc. Version 1.9 - 05.12.2014© 2014 imc Meßsysteme GmbH
6.19.4 Timing of the CAN message
The timing of the CAN messages is primarily determined by the SENT sensors. Following completed
reception and verification of a SENT messages, the CAN message is constructed and is thus ready for
transmission on the CAN-bus. The intended sending time for the CAN message to one SENT sensor is
independent of the sending time for the CAN message to a different SENT sensor. However, since in the
case of conflicts, multiple CAN messages can not be transmitted simultaneously but only sequentially, the
order of the CAN messages in time are mutually affected due to the limited capacity of the CAN-bus. In
particular, of very many sensors are connected, there will be an appreciable delay. The imc CANSAS-SENT
module transmits the CAN messages in the order of their respective intended sending times, or according
to the order of the incoming SENT messages.
The module can send a maximum of about 5 CAN messages per 1ms. The limit depends to a certain
degree mainly on the amount and complexity of FAST channels which have been parameterized. The
CAN-bus’ Baud rate must be set sufficiently higher to provide adequate transmission capacity on the bus.
6.19.5 Demands on the CAN-Bus
Example of demands on CAN capacities:
2 sensors are to be operated at a CANSAS-SENT module. Each delivers one SENT message every 3ms.
Each sensor has a (14-bit) pressure channel and a (10-bit) temperature. Also, the Serial Protocol is to be
evaluated.
On the CAN-bus, Standard-IDs are used at a Baud rate of 1 MBit/s.
The CAN message for a FAST channel has 14+10 = 24 bits of content. The module sends 2 such messages
within 3ms.
The evaluation of the Serial Protocol channels occurs after 16 FAST messages. Thus, every 3ms*16=48ms,
one SLOW message per sensor is generated. The SLOW message has 16 (control) + 8 (data) = 24 bits of
content. The module sends 2 such messages within 48 ms.
Total bus capacity demands for 47 bit overhead:
CAN-Bus capacity demands = 50291 [bit / s] / 1000000 [bit / s]= 0.050 = 5.0%
If the CAN-bus is overburdened, it is possible to use the parameter “Reduction”.
Note that the CAN-bus is not working at 100% capacity.
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