HP 41CL, thoughts...pics...part 1 (new pinout cap picture)



in one word "incredible".


well some more thoughts first. My donor is serial numbered 1933Axxxx (thanks Gene) making it a very early example with the "tall keys" keyboard. I used this donor because the case is pristine as is the battery module block and battery posts. The logic pca is dead, and I still haven't trouble shot it as yet. I also wanted to use the 'oldest version of 41C' that I had, just to contrast with the newest technology supplied by Monte.

After a thorough cleaning I was preparing to install when I read Monte's warning to READ the installation manual. Usually I don't! This time I did!!

According to Monte's installation instructions there is a warning about differing revisions on the LCD display. Revision F LCD panels need an installation of a .01uF from the OS1 pin out on the keyboard pca pinout. The ground is pin two of the keyboard logic pca. These two pins can be found at the LCD pinouts at the top of the keyboard pca also (see the pinout diagram at bottom of posting). This LCD pinout would be the easiest area to attach the 0.01uF capacitor. I will do that tomorrow.

Some shots:

the HP 41C serial numbered 1933A which is below the cutoff of 1954A. This means that I need to add the 0.01uF cap. Note the shinier keyboard bezel. Missing is the textured matte surface of later 41Cs.

Opening the case and placing the back aside, here are the sequential shots after removing the 41C original logic PCA:

Close up of the wired connector recommended by Monte and others:

The AC ball and springs connector as well as the original nuts that hold the oldest HP 41C logic pcas:

Close up of the original PCA obverse and reverse followed by Montes new 41CL:

You can see the quality in Montes 41CL, wonderful:

While you are here, clean the wire wrap connector with a soft natural brush stroking it the parallel to the wrapped wire. Also clean the battery-module block on all three sides. Insure continuity and that the pressure foam is still expanded. Clean contact surfaces of the keyboard pca. Dry and blow off dust when reassembling:

Now adding the serial bus port and double sided tape to hold the wire in place:

And now the CL in position:

Tomorrow, using Monte's emailed instructions and this excerpt from the HP41C manual illustrating the rev 7 LCD drivers attachment point and pinouts for the LCD:

Here is a close up on the revision 7 LCD drivers, as evidenced by the plastic caps as opposed to epoxy blobs:

As I said, standby for further!


edited as per typo indicated by Gene and other stuff.

Edited: 14 June 2011, 12:33 a.m.


In a 2009 issue of DATAFILE there was a short article entitled "Why the HP-41 can't be upgraded". I hope that this disproves that assertion.


it couldn't be done.

Thanks Monte for the 41CL. It feels the same as when I picked up my first 41C in 1981!


stepping out to get the phone to computer cable and the capacitor!


How´s that for an exaggeration then, although it´s taken 30 years and a clever design to break that myth.

The only thing that betters a 41 is... a faster 41!

Add to that the mass memory capacity of the 41CL and you´ll have the best there is...

Can´t wait to get mine installed, the anticipation is so good, I agree it´s like when I got my first 41C so many moons ago.


Oh oH, this is a hoot, so much to discover and automate, truly is the best non graphing calc around. Actually, got to fire up my plotter cus this does graph!


starting all over again with the 41!


Actually, the statement I made in Datafile was that the HP41 could not be upgraded _and maintain compatibility with all HP41 add-ons_.

I don't think the 41CL changes that. From what I understand, it can't run machine code from plug-in modules (only from the built-in flash memory on the 41CL board). I don't think it can access RAM on plug-in modules either. Yes, all the memory is built in to the HP41CL board, so there are very few times when you'd want to do either of these, but I can think of some.

The issue with periperhals is more complex. The 41CL claims to be compatible with all HP41 peripherals. I am quite sure it's compatible with all the standard ones, printer, HPIL, wand, card reader, etc. But the way HP41 peripherals work is to decode machine language instructions from the ROM (or ROM emualtor ;-)) on the ISA line. Unless the 41CL sends all ROM instructions over that line at the normal speed (which IMHO means it's not running any faster than a normal HP41), I claim I can design a peripheral that will work on an unmodifed HP41 but will fail on a 41CL, even if the ROM to control said peripheral is loaded into the 41CL flash memory.


The 41CL will run machine code from plug-in modules just fine, at normal 41 speed. This occurs when the MMU has a page set as "disabled", meaning that it uses external memory (normal 41 bus ROMs and such) rather than the 41CL flash. Your hypothetical peripheral should work just fine as long as its ROM is external.

If for some reason you do want to run your peripheral with its ROM code executing from 41CL flash, if you've followed the 41 bus protocol for peripheral instructions correctly, and not depended on instruction timing of non-I/O instructions, it should still work correctly, because the peripheral instructions always access the 41 bus. I would claim that a good design for a peripheral isn't dependent on the speed at which non-I/O instructions execute. However, it's certainly possible that you could design a peripheral for which that is not the case.


Considering that there hasn't been any new HP-IL peripheral designed in the last 20+ years (not counting the fantastic PIL-Box of course), it's interesting to see this hypothetical reason - just to prove or disprove the claim that it wouldn't be compatible with the CL. To me it's a moot point: as far as it counts, the CL is perfectly compatible with (what's left, used and known today) of the 41C system.

Speaking of which, the Pil-Box is also queued on the testing bench, will see how things fare..

Edited: 17 June 2011, 9:02 a.m.


The basic fact exists that for certainly all practical concerns, the HP 41 HAS been upgraded.

Clonix modules plugged in work fine. Hepax works fine. The Forth ROM works fine. HP IL peripherals work fine. The card reader works. The list so far goes on and on.

Monte's achievement is monumental.


Absolutely agree with that!


Hello Gene

Okay, there is a rub in all this. The experts, of which I am NOT one are looking or going to look at the addresses CLONIX uses when ROMS are loaded. These are NOT bugs with either the CL or the CLONIX. Both these work perfectly but at the moment there are conflicts on addressing and priortizing the access with CLONIX installed and the CL wins all the time.

My example looked like it worked with a couple of bugs. Probably the way I installed and then loaded created what appeared to be a perfectly function CLONIX CL CLONE ;-)

So here is what I did with a MEMORY LOST condition on the CL board (naturally occuring when you recieve the logic pca and install it.)

1 follow Montes' instructions to the letter while installing YFNS
to PORT 1 Lower.
2 now HPIL is available so cassette deck to calculator to fill
program memory with a suite of routines.
3 insert card reader to, of course, PORT 4 which is empty.
4 use RFL synthetic program to load DATA files into Extended
memory with the card reader.
5 use YFNS (CL) functions PLUG to load ROMs into PORTS.
7 see 6
8 Realize through explanation that the card reader when installed
takes over PORT 4 lower thus disabling any full page ROM residing
9 Read the manual again and realize that the ADVANTAGE module is
a full PORT! I loaded the ROM half page version of routines
based mistaking it to be the ADVANTAGE rom.

So it is a must to:


The treat here is you could write a routine called PCR and assign it to a key. Invoke it when you want to use the card reader and when done invoke UPCR.

01  LBL PCR (plug in card reader)
03 RTN
05 AON
06 "ROM?"
09 PLUG4

If it is always the same rom then replace lines 5 - 8 with the name of the rom in alpha. For example in PORT4 you want the full page ROM PPC. The program would look like:

01  LBL PCR (plug in card reader)
03 RTN
05 'PPCM'
06 PLUG4

It looked like the clonix would add two more full page ROMs to the calculator. In fact the reason it worked for me is the portion of the CCD and PPC that were required did not conflict. Cat II on closer inspection reveiled that only a third of CCD in clonix was available and the NAV1B never showed up at all.

In my defence I assumed that if a conflict occured I would see a complete failure of the CLONIX ROMS (not the CLONIX). They worked enough to sucker me in! That combined with the inability to find my CLONIX PIC program files to determine address loading and a lack of understanding of the intracacies of the 41CL/CLONIX addressing fooled me into thinking the combination worked.

So back to the PIC programmer to copy the CLONIX to the computer and determine the page addressing. Diego will also be looking into this along with Monte (emails).

Back to the machine:

I have the short routines loaded so that PORT4 can be swapped between the Card Reader and the CCD. I have PPC in PORT 3 and the full ADVANTAGE in PORT2. PORT1 lower has the CL YFNS functions while PORT1 upper has TREK.

Of course the beauty is that it so easy to swap ROMS in and out with the XEQ PLUG and UPLUG functions.

Well still playing and without ROM manuals that is a treat! The Museum DVD is a must!

Cheers, Geoff


Well it was an easy job to add the capacitor as explained in the following diagram:

Here is the capacitor in place, excuse my poor solder job:

Remember this is for the HP 41C Rev 7 LCD drivers with serial numbers less then 1954Axxxxx. Again these are recognizable by the plastic capped display drivers:

Also the early HP 41C may include the AC connector ball, springs and have the Logic pca attached by nuts:

Be careful removing the nuts as the posts tend to strip preventing the reuse of the nuts. If this happens a smaller diameter nut may be used or the following procedure can be followed. The tubing is the same tubing used on the card readers. It should be cut slightly longer then the screw post. It should sit proud of the top of the post as depicted below. This allows the bottom case via the case screws to push the new HP 41CL logic pca into the wire wrapped connector.

I have a couple of early 41C's modified this way and they work perfectly.


In order to avoid the 0.01uF (10nF) capacitor installation pick a donor with a serial number greater then 1954Axxxxx. If you do use an early one like I did then you need the cap.

Also, I had to add the 'time' module to take advantage of the timing functions of the CL board. My calc is now configured as follows:

1. serial port communications in port one
2. time module port two
3. IR printer module port 3 or HPIL
4. open for card reader if required and when I get more into
swapping the virtual ports then the card reader won't be

So far the Virtual Ports:

1. Port 2 CCD module full port
2. Port 3 PPC module full port
3. Port 4L (lower) TREK!

Thats all I have had time to do since I installed the cap. Well it will give me something to do at work! So I have HPIL from the tape drive my suite of programs, uploaded the data base to the extended memory, tried out turbo50 mode (as fast as the 42S) for some programs. Later, learn how to create my own module and load it into the board memory as a module and recall it via programming when required.

Lets put it this way, there is a LOT to learn AGAIN!!!!


Cheers. Geoff

Edited: 16 June 2011, 6:15 p.m. after one or more responses were posted


Geoff, you might be able to get along with only the CCD/OSX 4K rom instead of the entire 8K CCD module.

That would give you a 4K spot for some games or one of the game roms.


Thanks for the suggestion, I will try it although I realized that I have a clonix with the CCD, PPC and Nav1B modules in it.

Plugged it in and it runs with the CL perfectly. This opened up port 4 and 3.

So now the physical config is:

1. serial port connection
2. clonix (CCD, PPC, NAV1B)
3. Time
4. IRprinter, card reader, laser wand, hpil and its accessories
(swapping physically)

A small program is used to plug and unplug other roms in number 4 when the physical connections are removed. Alpha prompt along with the plug and unplug commands makes of the short program. Assigned to a key and the process is quick and easy.

LOVE THIS MACHINE (deja vu all over again)!


Edited: 14 June 2011, 10:51 p.m.


I'm sure that you have realized this, but I'll reiterate that the Port that is used for the serial connector is still open for a module image. The same thing is true for the Port that holds the time module, because the time module code uses page 5 rather than the Port address where it resides.


Thanks Monte;

Still have a lot of reading to do.

Some where I read that, but it dropped out in my excitement about the project. The serial port makes sense since it attached to the board and just uses the physical port one for wiring convenience. The time chip I did read but forgot.

Still playing with it, I sure hope my wife doesn't mind the key clicking while she is reading tonight.

Cheers, Geoff

Edited: 14 June 2011, 11:39 p.m.


If Retro-Gaming is the theme then you may also try the FunStuff module - I particularly enjoy doing a round of Fruit Machine or BattleShip while, say waiting at the doctor´s office, or even some turns of Roulette, Goufers, High Rollers, Pinball Machine, and Gene's own Yathzee's..

But pls. don't do that at the cockpit :-)


Speaking of which (the CCD/OSX that is), a new release is just available - see (buried under dozens of 34S threads):


It all comes together now, nice...


Answer to both postings:

we have a separate room above the first class galley that contains two bunks and two recliner seats each hooked into the video system. On the way to Sydney Australia from Vancouver with two complete crews (4) each group of 2 gets two breaks of 3 hours each.

That gives me lots of typing time on the book although now it looks like some of that will be exploring Monte's CL board!!!

I use quite alot of the CCD commands, especially the alpha display synthetics in my programs so it is a must. And of course a few PPC routines.

Trek is fun and I will give the others a try also!


I may have to push the release date on the book back another year (JUST KIDDING) ;-)


Hello Gene,

Haven't forgotten your 41C with bad ram, or the 67.

The short form of the ccd does not contain the synthetic alpha instructions I am calling upon. Not a problem as Deigos Clonic contains three full page roms in one physical port!

Cheers, Geoff


The short form of the ccd does not contain the synthetic alpha instructions I am calling upon. Not a problem as Deigos Clonic contains three full page roms in one physical port!

Hi Geoff, this intrigues me:- I thought the only "shortcoming" of the short form of the CCD (a.k.a. the CCD-OS/X) was the lack of displaying the file types during a CAT'4 execution, and this only for the extended ones (Matrix, Keys, Buffers)- it shows a blank for the file type.

Wouldn't it be because your machine has disabled the "synthetic alpha input", perhaps using function TOGLC (toggle Lower Case)?



Hello Angel,

yes it has something to do with alpha input. I created this program 5 years ago. It does a synthetic trick that used to take about 4 synthetic lines.

An alpha prompt occurs and the display arises in alpha mode. While typing the alpha response the original prompt remains. The CCDX version probably has a different rom number location for the command.

So I will determine the command (no manual) and then load CCDX and retype the command in place. If the command exists then it will have the new rom location attached to it.

In all it is just my ignorance with CCDX and the command I used.


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