Rockwell Automation 1771-SDN DeviceNet Scanner Module Installation Instructions - Manual (Page 18)

Publication 1771-IN014B-EN-P - September 2001
18 DeviceNet Scanner Module Catalog Number 1771-SDN/C
When using rungs like those in the preceding figure, the processor
alternates between the BTR and the BTW, waiting to request the next
block transfer until the previous block transfer is completed. When
only one block transfer is active at a time, it is considered a
single-threaded block transfer. Single-threaded block transfers operate
in an orderly sequence of read and write, however, they are slower
than the asynchronous method, shown below:
As soon as an asynchronous block transfer completes, it is requested
again, independent of other block transfers going to the same module.
If you have programmed many block transfers to the same logical rack
(especially a remote rack), you will fill the processor’s queue. In this
situation, the asynchronous method can result in irregular timing
between successive executions of the same block transfer.
For more information on programming and block transfers, refer to
your PLC-5 Programming Software Instruction Set Reference Manual,
publication 1785-6.1.
Using BTW to Send Outputs to the Scanner
The PLC-5 downloads output data to the scanner using block
transfer write instructions over the 1771 backplane. The scanner
module can update a maximum of 357 output words by internally
linking together six different sized block transfers as shown in the
following table.
Precondition
EN BLOCK TRANSFER READ
Rack
Group
Module
Control Block 3
2
0
DN
Data file
Length N11:0
62
ER
BTR
Continuous NO
EN BLOCK TRANSFER WRITE
Rack
Group
Module
Control Block
DN
Data file
Length ER
BTW
Continuous
Block-transfer rungs must be scanned for the transfers to occur. The
preconditions allow time-driven or event-driven transfers.
Precondition
BTR
enable bit
BTW
enable bit
BT10:0
EN
EN
BT10:0
3
2
0
N11:62
62
NO
BT10:5
BT10:5.EN