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CL3-E CANopen/USB Technical Manual

ConfigurationModbus RTU Parent topic: Modbus RTU

Process data objects (PDO)

Like done in CANopen a process image for input and output values can be configured in Modbus. This image contains only data values of one or more objects without any further information like length, index or sub-index. With transmitting one message a series of objects can be written or read.

Configuration

The image configuration is named "mapping" and is written in the following objects::
  • 3502h for the Modbus RX (Master → Slave) PDO Mapping
  • 3602h for the Modbus TX (Slave → Master) PDO Mapping
Both objects hold an array of 16 entries each. The sub-index 00 is telling the number of valid entries.

The objects 3502h and 3602h can be written with a message with the Modbus function code 2Bh.

Transmission

The data in the message are transmitted one after another without padding or alignment.

In case an alignment is necessary, additional "dummy objects" can be put in the message. Dummy objects will always transmit the value "0". These objects are listed in the following table.

Index Data type
0002h unsigned integer (8 Bit)
0003h unsigned integer (16 Bit)
0004h unsigned integer (32 Bit)
0005h signed integer (8 Bit)
0006h signed integer (16 Bit)
0007h signed integer (32 Bit)

The mapping is as follows:

  • PDO RX image starts at the Modbus register address 6000d (1770h).
  • PDO TX image starts at the Modbus register address 5000d (1388h).
The access is done with function code 17h reading/writing in parallel or with the command 03h, 04h, 06h, 10h on the corresponding RX/TX images.

Example

In this mapping the following objects are mapped:
  • 3602h:00h = 6 (6 values are mapped)
  • 3602h:01h = 60410010h (the object 6041h:00h, length 16 Bit is mapped)
  • 3602h:02h = 00050008h (Dummy-object 0005h:00h, length 8 Bit is mapped)
  • 3602h:03h = 60610008h (the object 6061h:00h, length 8 Bit is mapped)
  • 3602h:04h = 60640020h (the object 6064h:00h, length 32 Bit is mapped)
  • 3602h:05h = 60440010h (the object 6044h:00h, length 16 Bit is mapped)
  • 3602h:06h = 60FD0020h (the object 60FDh:00h, length 32 Bit is mapped)

After the mapping for the object 6061h:00h a dummy object was placed to align the following object 6064h:00h to 32 Bit.

RX message: the master transmits the following message to the slave:

TX message: the slave will answer the following message to the master:

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