Question:
What is better for communication performance purposes: one single block with 1,100 elements, or the same number of elements split into smaller blocks?
Solution:
Enabling this feature frees the user from having to consider block sizing. They can, for example, develop applications containing only PLC tags, because grouping optimization in reading operations is done automatically by both E3 and the driver.
Elipse SCADA, however, does not support superblocks; when using it with Modbus, the I/O block’s size must be considered.
Elipse SCADA
Automatic block splitting has been a feature of Modbus driver since its version 2.00. With this resource, the driver can now manage by itself the splitting of blocks larger than the protocol limits. Thus, the user will not need to consider whether they are exceeding the block’s maximum limit, because it is driver that will split these blocks into their proper sizes.
Still, knowing the protocol’s limits can help the user project optimized block sizes in Elipse SCADA, which does not support superblocks.
NOTE: Both superblocks grouping in E3 and the automatic block splitting in the driver require the device to support the limits established by standard Modbus. There are devices, however, that only support inferior limits. To make sure superblocks grouping and automatic block splitting will work in these cases, a new feature from version 2.03 will allow the driver to customize the maximum limit supported by the PDU (Protocol Data Unit). To apply this feature, access the driver’s Extra settings, check the option Customize Max. PDU Size on Modbus tab, and set the maximum amount of bytes supported by the device.
Limits Supported by the Protocol
Modbus protocol sets a simple data unit called PDU (Protocol Data Unit), which will remain unchanged in the protocol’s several modes and communication layers.
The whole communication frame, which includes the PDU and other additional fields from the header, is called ADU (Application Data Unit).
This means that a complete Modbus frame (ADU) can have a PDU whose maximum size is 253 bytes.
So, depending on the data type/Modbus function being used in the communication, the protocol will present the following limits for block elements:
For further information, please refer to Modbus’s official website.
