Could someone on the list explain why the RSLogix5000 programming software is NOT compatible with older versions of the ControlLogix firmware? In the past it has been the AB/RS modus operandi that new versions of a PLC programming package shall handle all firmware versions of all PLC's handled by earlier versios of the PLC programming package, as well as new firmware versions and/or PLC's that have been added. This is apparently not the case with RSLogix5000. Each revision of RSLogix5000 is only compatible with a particular revision of ControlLogix firmware. If it is necessary to work online with an older revision of CL firmware, only a particular version of RSLogix5000 may be used successfully. Currently, to handle multiple CL firmware revisions, it is necessary to: 1) Uninstall and reinstall between different RSLogix5000 revisions. 2) Swap between hard drives containing different revisions of RSLogix5000. or 3) Configure your computer for multiple boots of NT 4.0, each with a different revision of RSlogix5000. The current worst case scenario requires access to ten different revision levels of RSLogix5000 software to properly deal with the various revision levels of the CL firmware (per AB publication 9324-AP001A-EN-E - December 2000 http://www.ab.com/manuals/cl/9324-ap001a-en-e.pdf). This will apparently continue to get worse with each new release, unless they change their approach. AB GTS indicated to me that a fix, or workaround, for this situation is in the works, but could not tell me when to expect it. From what I know, it could be weeks or years away. If I am incorrect in my assessment, please let me know. I would rather be wrong than have to deal with things as I have described them. If I am correct, AB/RS erred when they chose not to use their traditional backwards compatibility approach, IMHO. I have been an advocate of AB PLC's in the past but am disappointed by the state of affairs with the CL. I might be more understanding if I knew the reason why they created this situation. Speaking for myself, not my employer, Troy Stearns Controls Engineer Martin Automatic, Inc.