Data Transfer Optimization

This section describes the data transfer optimization implemented for the Modbus scan task. Optimization is the means by which the scan task seeks to minimize the load on the communication channel by making scan device requests more efficiently.

The aim of optimization is to include as many scan input records in a single request as possible, thus reducing the communication overhead.

Writes are not optimized since writes to a scan device typically occur much less frequently than polls or reads.

Follow these guidelines, whenever possible, to minimize communication overhead:

• keep modbus function together in the scan input table, for example, group all read input coil points together,
• order scan input table register in ascending order,
• eliminate gaps in register; if one/two registers are not required, configure them into the scan input table anyway (make sure they are enabled),
• assign the same poll type to groups of records that have consecutive register,
• groups exception record records together following guidelines 1 to 4 above.

Digital inputs (ie. read coils and read statuses) will be optimized to be read in a single request if the digital is within the current byte or within the next byte in the response.

The scan task will break a group of scan input records if one of the following conditions exists:

• different modbus function
• for digitals, the modbus register is not within the current byte or the next byte
• for input/holding registers, the modbus register is not the next register
• data size is different
• the number of registers is different