With the PLC-5 controllers, a Series change was a change in hardware as well as some core functionality.
You can use firmware chipsets to upgrade within a Series, but you can't upgrade from one Series to another. RA only offers PLC-5 upgrades via their remanufacturing service.
There's a nice concise PLC-5 Enhanced and Ethernet controller history in the RA Knowledgebase, as Answer ID # 18946 (Access Level: Everyone).
The major feature added in Series E was support for 2,000 Program Files, as well as some changes to the PID instruction.
The PLC5 controller 1785-L40E Series D was called an Enhanced controller, "CE Watermark". It had 48K of user memory. The PLC5 1785-L40E Series E "2000 files/extended forces" had the same amount of memory, but allowed up to 2000 files. Both of these are available in "Ehternet" versions, which have an Ethernet communications port as well as the DH+ and RIO ports.
- Support multiple baud rates for DH+ on Channel 1A. Channel 1A baud rate, either 57.6KB or 230.4KB is determined by SW1-7 on the PLC. - Supports 2,000 program files. - Provides PID zero crossing deadband disable option. - Provides PID bias back calculation disable option.