Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 358)

Publication 1785-UM012D-EN-P - July 2005
F-10 Troubleshooting
Unexpected Operation
when Entering Run Mode If unexpected operation occurs whenever your controller enters run mode, be
sure to examine the prescan operation of the instructions in this section.
These instructions execute differently during prescan than they do during a
normal scan.
The prescan function is an intermediate scan between the transition from
program to run modes, during which all rungs are scanned as false. The
prescan examines all ladder program files and instructions and initializes the
data table based on the results of the program.
For example, a subroutine that is called infrequently may contain a bad indirect
address and generate a major fault. However, many normal program scans
may occur before the major fault is actually generated. Prescan provides the
opportunity for the controller to examine the program for errors such as this
before transitioning to Run mode.
Instructions with Unique Prescan Operations
Use the table below to track prescan operations that deviate from normal
instruction operation.
This
Instruction: Executes These Actions During Prescan:
ARD If the EN bit is set and the DN and ER bits are cleared, then the control
word is cleared. If either the DN or ER bit is set, then the EN bit is
cleared and the DN bit is set.
ARL
AWT
AWA
ACB
ABL
AHL
BTR All non-user configuration bits 15, 14, 13, 12, 10, and 9 are cleared (for
both INT and BT file types).
BTW
CTU The CU/CD bit is set to prevent a false count when the first run-mode
scan begins.
CTD
EOT This instruction is skipped so all ladder instructions can be prescanned.
FFL The EL bit is set to prevent a false load when the first run-mode scan
begins.
LFL
FFU The EU bit is set to prevent a false unload when the first run-mode scan
begins.
LFU
FND This instruction is skipped so all ladder instructions can be prescanned.