1541 Ultimate Firmware v1.5:

- Show all entries in a D64 when loading directory through software IEC- Show all entries in a D64 when loading directory through software IEC

Known issues:

- CD:/ of the software IEC mode is broken- MOUNT: is partly broken (1541-bug-workaround was removed because we forgot why wedid that! Temporary solution: Issue initialize command to 1541 after mounting a D64: @8, @I)

Fixed from 1.5beta

- The menu waits until you release the keyboard before exiting.

1541 Ultimate Firmware v1.6

- Reads from the Ethernet chip have been fixed. The unit now correctly receives packets.

1541 Ultimate V1.7 beta Firmware Download

Installation instructions: 

1) Make sure that in your current configuration, the "application to boot" is set to "appl.bin" (the default).

2) Unzip the zip file into the ROOT of an SD-card, of which you have made sure that the 1541 Ultimate can read it.

3) Place the SD-card in the 1541U, and make sure the 1541U is correctly inserted in a C64/C128 expansion slot.

4) Turn on the C-64 and watch the screen output.

5) Turn off the C-64 when the update is finished.

In comparison to 1.6, the following has been changed:

• The freezer has been made more robust. (Needs more testing)
• Minor fixes in the IEC interface (but not yet satisfactory)
• SID player has been added
• Epyx Fastloader cartridge has been added
• Support for custom carts has been added (8K/16K, as well as existing carts)
• Support for (custom) Ocean and System3 cartridges added.

This is a BETA release.

1541 Ultimate Firmware 1.72b [unofficial] with WVLs config save flash indication and P00 support, patched with the Ultimate-freezercode from 2.0 firmware (demos like Another Beginning and We Are New now works) and added the (Blu)REU-imagesupport. by Tommi Lempinen

As far as I can tell, this is the latest and last firmware update for the original 1541 Ultimate as of August 2019.  This firmware was released in 2012.

1541 Ultimate Firmware v2.5 [unofficial] with KCS power cartridge support, 1 drive and sound

Released in November 2013, this firmware for the 1541 Ultimate I is the latest unofficial firmware available.  More information on this firmware by Obliterator918 can be found here.

Found this older user guide for Gideon Zweijtzer's 1541 Ultimate.  Not sure if there is a newer one, but this one has a LOT of good info in it.  If you have on of these awesome cartridges for the Commodore 64, you'll want this guide.