Allen-Bradley PLC-5 User Manual page 221

The 1771 read command type of the CIO instruction operates in the same
manner as the BTR instruction; the 1771 write command type of the CIO
instruction operates in the same manner as the BTW instruction. For
simplicity, the following descriptions and examples of extended forcing refer
to the BTR instruction (for BTR and 1771 read command type of CIO
instructions) and the BTW instruction (for BTW and 1771 write command
type of CIO instructions).
You program block-transfer instructions in the same manner, regardless of
whether you configure the data file in the extended for configuration table.
The following figure shows how block-transfer data table files are updated
during housekeeping.
BTR
Data File
each housekeeping period
BTW
Data File
When you use extended forcing, you affect the way your programmable
controller system operates, Before you begin to use this forcing feature, read
this entire section to understand the effects.
Any block-transfers or data table locations included in
ATTENTION
the extended force configuration table will be affected
regardless of whether forces are enabled.
Do not use BTR data tables files to store
ATTENTION
non-block-transfer data. All non-block-transfer data
that you include in the extended force configuration
table as read data will be forced to zero during
housekeeping at the end of each program scan. If your
ladder program expects values other than zero for this
data, unpredictable machine operation could result.
Programming Considerations
BTR instruction execution
BTR
BTR Module
Data Buffer
BTW
BTW Module
Data Buffer
BTW instruction execution
Publication 1785-UM012D-EN-P - July 2005
13-3