Can Messages - Siemens SIMATIC PN/CAN LINK Operating Instructions Manual

5.2.3

CAN messages

In "CAN transparent" mode, data can be exchanged using configurable or programmable
messages, both of which can be used in parallel.
Configurable CAN messages
The configurable CAN messages have a fixed message ID and a fixed length. The "Transmit
message" and "Receive message" submodules are available in TIA for configuring (see
Configuring CAN transparent (Page 129)).
Messages in standard format CAN 2.0A (11-bit message ID) and in extended CAN format 2.0B
(29-bit message ID) can be used.
Note
No provision is made for flow control. That is, if data arrives via CAN messages faster than it
can be sent via PROFINET, intermediate values may be overwritten. The same happens in the
opposite direction when data are changed faster by PROFINET than they can be sent via CAN.
Changes on the CAN bus may not be visible in this process.
The operating principle of the fixed CAN messages is based on data mapping. This means a
loss of intermediate values is acceptable.
Fixed messages can be sent in the following two ways:
• When the data to be transmitted changes
• Time-controlled, that is as soon as the configured cycle time has elapsed.
With this transmission type, the user data of the CAN messages is transmitted between the S7
controller and the PN/CAN LINK in the cyclic IO image.
Programmable CAN messages (proxy)
CAN messages for which the message ID and the length (max. 8 bytes) is not determined by
the S7 user program until runtime can be used in "CAN transparent" mode.
With this transmission type, protocol information in addition to the actual user data is
transmitted between the S7 controller and the PN/CAN LINK in the cyclic IO image.
The message ID format (11 bits or 29 bits) is determined by bit 29 in the message ID. If this
bit is set, the message ID format with 29 bits must be used.
The sending and receiving of CAN messages from the S7 user program is possible using
transmit and receive proxy modules. See: Cyclic data exchange between controller and
PN/CAN LINK for programmed CAN messages (Page 71)
SIMATIC PN/CAN LINK
Operating Instructions, 03/2022, A5E39895388-AC
Functions
5.2 CAN transparent
67