GridConnect CAN232 FD, User Manual

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CAN232 FD User Manual 3-19
3.2 Using Message Filters
In command mode, the CAN232 FD supports message filtering through the use of identifier ranges or masks
and filter values, providing the ability to receive only a sub-set of all the possible CAN identifiers. This can
greatly reduce the required serial data bandwidth on a busy network by only allowing certain messages to be
received. Required serial bandwidth can be further reduced by using limiters.
To use the filters, they must be enabled, their type must be specified, and appropriate ID values must be
assigned. This configuration is done while in the configuration mode.
3.2.1 Defining Filter Entries

CAN messages can be filtered according to a message filter list.

The list defines which identifiers will be received and which will be ignored.

Each filter in the list can be enabled or disabled separately.

Definitions can be either a single ID, a pair of IDs, an ID range or an ID mask and match value.

A filter can be inclusive (allow to pass) or exclusive (discard).

Standard (11-bit) and Extended (29-bit) IDs can be defined.

Definitions must not conflict nor overlap.

Filtering can be enabled or disabled under the Command settings.

When disabled, all CAN messages are received regardless of the definitions in the list.

When enabled, only messages which match filter definitions in the list will be passed through.

An additional limiter can be set for messages that pass through the filter by dividing the number of
messages (eg. 1 out of 20) or keeping a minimum frequency (eg. 1 every 100 ms).

An empty filter list that is enabled will receive nothing, but still permit transmission (making the
unit effectively TX only).

The maximum number of filter definitions possible is ten (10) each for Standard and Extended IDs.
3.2.2 How Filtering Works
Whenever the CAN232 FD receives a CAN message from the network, it checks to see if any filters are
enabled. If none of the filters are enabled, it assumes no filtering should be performed and outputs the
command message format to the serial port. If any filters are enabled, the CAN232 FD will attempt to match
the received message to each enabled filter in order. At the first successful match, the message is accepted
(if inclusive) or discarded (if exclusive). If accepted the message is checked against any limiters then
converted to command message format and output to the serial port. If no matches were successful, the
message is discarded.
Note: The output format depends on mode : ASCII or BINARY
3.3 Tunnel Mode
Tunnel mode is used to transparently transfer serial data over the CAN bus. In previous versions of the
product this feature was referred to as virtual circuit (VC) mode. In Tunnel mode, the CAN232 FD
establishes a full-duplex virtual serial link between itself and an application on another CAN node. A serial
device can transparently send and receive serial stream data on the CAN232 serial line and the CAN232 will
route the data over CAN to the configured target device.
A pair of CAN232 devices can be utilized to create a virtual serial link. By providing a virtual serial link
over the CAN network, an application can exchange data with a serial device in a network-transparent
fashion, using existing CAN network cabling as a data link (Figure 4).