Prosoft-technology MVI69-MCM Manual de usuario descargar pdf (Pagina 116)

Reference MVI69-MCM ♦ CompactLogix or MicroLogix Platform

User Manual Modbus Communication Module

Page 116 of 167 ProSoft Technology, Inc.

March 22, 2011

5.4.3 Read Block and Write Block Transfer Sequences

The Read Block ID is an index value used to determine the location of where the

data will be placed in the processor controller tag array of module read data. The

number of data words per transfer depends on the configured Block Transfer

Size parameter in the configuration file (possible values are 60, 120, or 240).

The Write Block ID associated with the block requests data from the processor.

Under normal program operation, the module sequentially sends read blocks and

requests write blocks. For example, if the application uses three read and two

write blocks, the sequence will be as follows:

R1W1→R2W2→R3W1→R1W2→R2W1→R3W2→R1W1→

This sequence will continue until interrupted by other write block numbers sent by

the controller or by a command request from a node on the MODBUS network or

operator control through the module’s Configuration/Debug port.

The following example shows a typical backplane communication application.

If the backplane parameters are configured as follows:

Read Register Start: 0

Read Register Count: 480

Write Register Start: 480

Write Register Count: 480

The backplane communication would be configured as follows:

Database address 0 to 479 will be continuously transferred from the module to

the processor. Database address 480 to 959 will continuously be transferred

from the processor to the module.

The Block Transfer Size parameter basically configures how the Read Data and

Write Data areas are broken down into data blocks (60, 120, or 240).

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Prosoft-technology MVI69-MCM Manual de usuario Pagina 116