Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 222)

Publication 1785-UM012D-EN-P - July 2005
13-4 Programming Considerations
For BTR instructions using non-configured data tables, the .DN bit indicates
when data is valid in the BTR data file. When you configure files in the
extended force configuration table, the .DN bit indicates that the data is in the
BTR data buffer. The BTR data is not forced and moved into the BTR data file
until the next housekeeping period. Delay using the BTR data until the scan
after the .DN bit is set.
For BTW instructions using non-configured data tables, the data that is in the
BTW data file when the block-transfer is enabled is transferred. When you
configure files in the extended force configuration table, the data that in the
BTW data buffer when the block-transfer is enabled is transferred. Any new
data in the BTW data file is not forced or moved into the BTW data buffer
until the next housekeeping period. Delay enabling the BTW instruction until
the scan after the BTW data file is updated.
Increased Program Scan Time
When you use the extended forcing feature, program scan time increases
proportionately to the number of words you configure in the extended force
configuration table. The amount of increase depends on whether you enable
or disable forces. Typical increases in program scan time when you configure
data table files in the extended force configuration table are:
I/O Force Privileges
The I/O forcing privilege lets you enable, disable, or clear all forces in the
controller. This privilege now includes extended forcing.
Extended forcing reads force data in a read data file; extended forcing writes
force output data, leaving the write data file in its original state.
When Forces are Scan Time Increases by this Much
Per Word Per 1000 Words
enabled 0.003 ms 3.0 ms
disabled 0.0015 ms 1.5 ms
IMPORTANT
Forces are held by the controller (and not the personal
computer). Forces remain even if the personal computer is
disconnected.