Modbus Slave Mode - ICP DAS USA I-7530A-MR-FD User Manual

5. Modbus Slave Mode

The I-7530A-MR-FD, Modbus RTU to CAN converter, supports the Modbus
RTU protocol. It can act as a Modbus RTU slave on the Modbus network. There
are some mechanisms for data-exchanging between the CAN register and the
Modbus RTU register as the figure at the next page.
In the Modbus Input Register, according to the different purposes these
register are divided into three fields, "Normal CAN Message Field", "Specific
CAN Message Field" and "Module Status Field". When a CAN message
received from the CAN network, the I-7530A-MR-FD will check if the Specific
CAN Message filed is used or not. If it is not used, this CAN message will be
stored into the "Normal CAN Message" field. This filed is similar with a kind of
FIFO (first-in first-out buffer). Users can only read this field with the start
address of this field by applying the Modbus commands. After users read the
CAN messages from this filed, the rest unread CAN messages will be moved to
the buffer with the start address of this field. This field can store maximum 200
CAN messages. Therefore, if the unread CAN messages exceed 200 records,
the data is lost.
If the Specific CAN Message filed is used, the CAN messages which are
marked in the specific CAN message table of the Utility tool are directly moved
to the Specific CAN Message field. CAN messages with different CAN IDs will
be stored in different parts of the Specific CAN Message field. Users can set
maximum 100 different CAN ID of CAN messages .Besides, a kind of CAN ID
only has one record buffer. If there are two CAN messages with the same ID,
the later will over-write the former. Therefore, the Specific CAN Message filed
always keeps the newest information of the corresponding CAN messages with
the specific CAN IDs.
If a CAN message is sent to a CAN network from a Modbus network via the
I-7530A-MR-FD, the CAN message will be temporarily stored in Output
Register and not be transmitted until the CAN bus idle. The Output Register is
only one message buffer. If the data overrun is happened, users will get an
error code for replying. Users can also use Modbus RTU command to read the
CAN message transmitted before. It is helpful for checking the last sent record.
I-7530A-MR-FD Modbus RTU to CAN Converter User's Manual (Version 1.0, 08/2022) ------------- 63