1756-L6xS
firmware version v. 17 - 20
battery back-up of user application
1756-L7xS vs. 1756-L6xS
firmware version v. 20+
non-volatile memory is SD card instead of CompactFlash
Replaceable Energy Storage module for battery back-up of user application
1756-L8xES vs. 1756-L7xS
firmware version v. 31+
Non-replaceable Energy Storage module for battery back-up of user application
Safety Signature has changed instead of 32-bit hexadecimal value it is a 32 character hexadecimal value stored as a SINT(33).
Built-in Ethernet port
Task Monitor Tool do longer works. To get similar data there is a web page on the controller itself.
See also:
http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm100_-en-p.pdf
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um001_-en-p.pdf