Notes Regarding Block Transfer Programming - Allen-Bradley 1203 Series User Manual

5–8 Using Block Transfer Messages
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Notes Regarding Block Transfer
Programming
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Figure 5.6
Due to the asynchronous nature of the ControlLogix platform, input data may change during a program scan.
This rung makes a local copy of the Block Transfer Status word to ensure proper synchronization
between the ControlLogix program and the RIO Scanner.
This rung sends a BTW to the drive if the drive's RIO adapter is ready to accept a BTW and no response
from a previous BTW is available to be read with a BTR. The data instructs the RIO adapter to send
a SCANport message to the drive. When this message has completed, the response can be read with a BTR.
The BTW Available and BTR Available bits will not be set until the SCANport message has been completed.
BTW Available BTR Available
GD1_LocalCopy.13 GD1_LocalCopy.10
This rung sends a BTR to the drive whenever the drive's RIO adapter indicates that it has a response
message available from a previous BTW. The data read contains the response to the SCANport
message sent by the previous BTW.
BTR Available
GD1_LocalCopy.10
• A Block transfer subroutine can be used to transfer more data
than can be moved in a single block transfer. If this is done, the
block transfers must be carefully sequenced so that one Block
Transfer Write and one Block Transfer Read occur for each
portion of the sequence. One method of doing this is to set a latch
bit to enable the Block Transfer Write and unlatch this bit when
the Block Transfer Write is completed. When the Block Transfer
Read completes, the program can then set up the data for the next
transfer.
• The status bits from the BTW and BTR Control files (.EN, .DN,
.ER) may change at any time during a program scan. If they are
used by the program they should be copied to a file and the
program should use the copied versions.
Example for a Logix5550
Move
Source
Dest
MSG
Type - Block Transfer Write
Message Control PLUS_BTW_Control
MSG
Type - Block Transfer Read
Message Control PLUS_BTR_Control
Make a Local Copy
of the 1203-GD1's
Block Transfer
Status Word
MOV
PLUS_IO:I.Data[0]
2#0000_0000_0000_0000
GD1_LocalCopy
2#0000_0000_0000_0000
EN
... DN
ER
EN
... DN
ER