Omron CJ1W-CORT21 Operation Manual page 42

1 Features and System Configuration
expected to occur; the distance between an edge that occurs outside of Sync_Seg and the
Sync_Seg is called the phase error of that edge. A signal-edge is expected to lie in this segment,
and is used to synchronize the bus input to the system clock. The Propagation Time Segment
Prop_Seg is intended to compensate for the physical delay times within the CAN network. The
Phase Buffer Segments Phase_Seg1 and Phase_Seg2 surround the Sample point.
 Sample Point
The sample point is the point of time at which the bus level is read and interpreted as the value of
that respective bit. Its location is at the end of Phase_Seg1. The sample point is set as a percentage
of the total bit time: 0% - 100%. It can be set indirectly or directly for the User Defined CAN Unit. It is
set indirectly with the baud rate switches on the front of the Unit, or directly using a message com-
mand (2909).
OSI Layer 2: Datalink Layer
The CAN bus access protocol is implemented by OSI layer 2. This protocol also includes data security
and the handling of the transmission protocols and messages. The datalink layer is layer 2 of the
ISO/OSI reference model and is built with two sub-layers on top of each other: the Logical Link Control
(LLC) sublayer and the Medium Access Control (MAC) sublayer.
 MAC
The Medium Access Control (MAC) specifies the procedures which determine when a device is per-
mitted to transmit data. Information from transmitters to receivers are passed in data frames. The
MAC sublayer represents the kernel of the CAN protocol. It presents messages received from the
LLC sublayer and accepts messages to be transmitted to the LLC sublayer. The MAC sublayer is
responsible for Message Framing, Arbitration, Acknowledgement, Error Detection and Signalling.
 LLC
The LLC sublayer is concerned with Message Filtering, Overload Notification and Recovery Man-
agement. For a detailed description of the MAC and LLC sublayer refer to Bosch CAN Specification
Version 2.0.
1-8
Arbitration field Control
Field
S R I r
O 11 bit IDENTIFIER T D 0 DLC 0 - 8 bytes
F R E
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
Data Field CRC Field Ack End of
Field Frame
15 bit CRC
Int Bus idle