decompiler issue

I am trying to upload from an 1747-L40C
using PCMK card and1784-PCM4/B cable I have several
of these bricks controling the spur line to main line
conveyor each has a DCM card communicating to SN on
main line. The unit is functioning properly however I can
see it in Linx but get the DECOMPILER (UNKNOWN) when ever I attemt to upload program
I am using RS500 8.10.00 with Linx Classic 2.56.00.13
Aall other units respond correctly, the programing is from 1979.

See link below. I don't have a tech connect contract so I don't know what they say.

http://rockwellautomation.custhelp.com/app/answers/list/kw/DECOMPILER (UNKNOWN)/r_id/113025/search/1

As Mickey's links point out, some programs in the very old fixed I/O SLC-500 controllers and 5/01 and 5/02 controllers can't be uploaded (which involves decompiling) by RSLogix 500.

The compiler in the very old APS (prior to version 4, so about 20 years ago) had a handful of opcodes that got missed in the transition to the Windows compiler.

If you use the DOS-mode decompiler/upload utility linked to in the Knowledgebase and then download again using RSLogix 500, the programs will be fully accessible by RSLogix 500.

Thanks, Rockwell sent me the program to use to upload as you said.
never had this problem before with any of the other units.