# SLC 504 OS versions **URL:** \ **Category:** Archived Topics\ **Created:** [April 29, 2015, 5:24pm UTC](https://forum.inductiveautomation.com/t/slc-504-os-versions/9968 "2015-04-29T17:24:17Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Henry](https://avatars.discourse-cdn.com/v4/letter/h/df705f/32.png) [@Henry](https://forum.inductiveautomation.com/u/Henry)\ **Post date:** [April 29, 2015, 5:24pm UTC](https://forum.inductiveautomation.com/t/slc-504-os-versions/9968/1 "2015-04-29T17:24:17Z")
Could someone explain the SLC 504 OS versions, series A to E or firmware? I have an old SLC504 that says it is an OS401. This CPU seems to be older than Google searches where used hardware is showing up as series A to E. Do the newer versions just have bug fixes and some new instructions? Can I use RSlogix to read my firmware version? Thanks ---
**Author:** ![AlThePal](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/althepal/32/3603_2.png) [@AlThePal](https://forum.inductiveautomation.com/u/AlThePal)\ **Post date:** [April 30, 2015, 3:18pm UTC](https://forum.inductiveautomation.com/t/slc-504-os-versions/9968/2 "2015-04-30T15:18:10Z")
AFAIK, OS401 is the latest version and came in series A, B or C, with C being the most recent. Each series then had different firmware revision numbers (FRN) with FRN 11 (I think) being the latest in November 2012. You can get a kit to upgrade the processor as described [here](http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1747-in007_-en-p.pdf) but I’m not sure how recent your processor must be to upgrade it to the newest firmware. Have a look at [this release note](http://literature.rockwellautomation.com/idc/groups/literature/documents/rn/1747-rn013_-en-p.pdf) for information on the kind of things fixed. I think you also have to match your version of RSLogix 500 to the version of controller firmware, but I don’t know whether you can use it to read the firmware version or not. ---
**Author:** ![Henry](https://avatars.discourse-cdn.com/v4/letter/h/df705f/32.png) [@Henry](https://forum.inductiveautomation.com/u/Henry)\ **Post date:** [April 30, 2015, 4:27pm UTC](https://forum.inductiveautomation.com/t/slc-504-os-versions/9968/3 "2015-04-30T16:27:12Z")
thank you