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HP-41CL (replacement CPU board) Project


The 41CL is an upgrade to the venerable HP-41, created by replacing the CPU board in a HP-41C/CV/CX "fullnut" with a new circuit board.

The NEWT (Nut, Expanded With Turbo) microprocessor used in the 41CL provides a number of new features surrounding a clone of the CPU originally used in the 41c series. These new features, particularly the Memory Management Unit (MMU), are probably more complex than the typical HP-41 user is used to dealing with.

The 41CL attempts to hide this complexity from the typical user, while still providing full control over the hardware to the advanced user. This is accomplished by including a special set of functions, much like the Extended Functions present in the HP-41CX.

Current Status

Last update: 16-Aug-2016

Another batch is available. See the project home page (below) for details.

One word: Amazing!


The 41CL takes advantage of modern technology to significantly add to the capabilities of the HP-41 system. In particular, the 41CL provides the following features:

• All features of an HP-41CX except for the Time Module. CX Time functions (the software) are included, but a Time module plugged into a Port is required for full timer functionality.

• Full 600-register Extended Memory is built in.

• Roughly 300 plug-in module images are built in. Functions are included to allow these images to be virtually plugged into a calculator Port and unplugged from a calculator Port.

• Turbo mode, which allows the calculator to run at up to 50X normal speed. Actual values available are 2X, 5X, 10X, 20X and 50X.

• 16 empty pages (4K in length) of Flash memory are available for non-volatile storage.

• 121 pages (4K in length) of RAM are available. All RAM is continuously powered.

• A sophisticated Memory Management Unit (MMU) allows full access to the large physical memory.

• Full bus compatibility for the Ports, allowing the use of any peripheral designed for the HP-41 system.

• A full-duplex serial port is available when the optional serial connector is used. This optional connector uses a 2.5mm stereo jack mounted in a blank port cover.

With these features, however, come some drawbacks:

• Power consumption is higher, at least while the calculator is off or in light sleep (between keystrokes). Where the original HP-41 required about 10uA while off, the 41CL requires about 110uA. This will lead to reduced battery life.

• The original HP-41 could retain memory contents for several minutes while the batteries were changed. Because of the higher current consumption, the 41CL only retains the memory contents for a few seconds while the batteries are out. For this reason, you should probably have an extra battery holder ready to go when changing batteries.

• The advanced technology used in the 41CL is a double-edged sword. The Flash memory, as well as the programmable logic devices used to implement the NEWT microprocessor, only guarantee data retention for 20 years.


Getting Started


Technical Information

Note: some of the sections below need updated, so feel free to contribute :)

41CL Discussion Forums

The following simple discussion forums have been created for users of the 41CL. All of the forums below are in wiki format, so posting is easy and quick.

If you need to create a new top-level discussion topic, use the “New thread” box below.

41CL Forum 2014/10/10 08:15 roger schoenthal 10 Comments
Serial port to computer RS232 2014/10/02 03:45 roger schoenthal 1 Comment
Memory address and paging 2014/10/02 03:45 roger schoenthal 0 Comments
CL ROM and Routines 2014/10/02 03:45 roger schoenthal 4 Comments
CL patches and bugs 2014/10/02 03:45 roger schoenthal 0 Comments


  • No downloads available yet.

Code Snippets

41cl/replacement_cpu_board.txt · Last modified: 2016/08/16 10:27 by montedalrymple