Installation manual

Publication 1747-RM001G-EN-P - November 2008
Application Specific Instructions 7-23
Read Program Checksum
(RPC)
The Read Program Checksum (RPC) instruction copies the checksum of the
user ladder program from either the processor’s RAM memory or from the
installed memory module into the designated destination integer file location.
The program checksum is a 16-bit value that is calculated for the entire ladder
logic image, excluding the data table values. The checksum changes any time
program edits are made. Therefore, the RPC instruction can be used within the
ladder logic to determine whether online edits have changed the checksum
from a known value or can verify the program in the memory module matches
the program in the processors RAM memory. If a mismatch is detected,
additional ladder logic may trigger an alarm bit for a higher level HMI to
evaluate or even fault the processor so that it cannot run with a modified
program.
This instruction requires RSLogix 500 software, version 7.10 or higher, for
programming.
Enter Parameters
• Proc/Mmod selects the source of the program checksum;
PROC-MEM for processor RAM memory or MEM-MOD for memory
module.
• Destination is the integer file address where to copy the selected
program checksum.
Output Instructions
Fixed SLC
5/01
SLC
5/02
SLC
5/03
SLC
5/04
SLC
5/05
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