Rockwell Automation 1771-DB BASIC MODULE - Manual (Page 78)

Chapter
Programming Block-Transfers 5
5 -4
Block-Transfer Write Buffer
The BASIC module processor maintains a block-transfer-write (BTW)
buffer containing the values of the last BTW sent by the PLC processor.
Use CALL 4 to set the BTW word length. Transfer data to the BASIC
module’s BTW buffer with CALL 6 or CALL 3.
Block-Transfer Read Buffer
The BASIC module also maintains a block-transfer-read (BTR) buffer that
is the value of the next block, read by the PLC processor. Use CALL 5 to
set the BTR word length. Transfer the data to the BASIC module
processor BTR buffer (for subsequent transfer to the PLC processor) with
CALL 7 or CALL 2. You should complete the building of the read buffer
before initiating its transfer.
Important: Use CALL 4 and CALL 5 once, immediately after power-up if
possible. CALL 4 sets the BTW block length. CALL 5 sets the BTR
block length. Failure to follow these guidelines could cause random
lockup of the BASIC module program.
Block-Transfer Buffers