Rockwell Automation 2706-MB1 MESSAGEVIEW 421 SOFTWARE MANUAL - Manual (Page 109)

5–20 Communications Overview
Publication 2706-817
Helpful Hints for Block Transfers
Keep these suggestions in mind when using block transfers:
•
Read and Write Blocks can use the same address.
•
Read and Write Blocks may overlap partially or completely.
•
Since Remote I/O scanners usually make only one block transfer
in one I/O scan:
– improve response times by compacting the data into the
fewest possible blocks (no unused bits)
– if the Function Key Return Message Number global attribute
is enabled, make sure that the tags for function keys are in the
same block as the Function Key Return Message Number tag.
Example: Block Transfer Read
In this example, the MessageView 421 terminal is assigned to rack
#3 and its starting module group is 0. A Numeric Variable Entry
variable has the address N7:2, and its value is read by the logic
controller in a part of the data block being transferred.
BLOCK TRANSFER READ
Rack 03
Group 0
Module 0
Control Block N36:0
Data File N7:2
Length 4
Continuous N
➀ The operator enters 30 as the motor
speed in the Numeric Variable Entry.
➁ Using information supplied by the Communication
Setup utility, the MessageView determines the
position of the data and the block size.
Entry variable with
Tag Name “Motor_Speed” to
Address N7:2
➂ The MessageView terminal places the
data in the first word of a 4 word block.
➃ The PLC reads the 4 word block of data
and stores it at the Data File address.
BTR
(EN)
➄ The value (30) entered on the MessageView
terminal display is now stored at N7:2.
Read From
MessageView
Data To PLC
Word 1 30
Word 2
Word 3
Word 4