41CX display contrast


One difference between the CX Fullnut and the Halfnut versions are the display segment widths, and the lower visibility or apparent contrast of the thin Halfnut segments. I do remember that the display contrast of my old CX Fullnut that was stolen was adjustable somehow. I've read very carefully both manuals for my replacement CX Halfnut, 'Synthetic Programming' book by Wickes, 'Calculator Tips & Routines' book by Corvallis Software, and the PPC Rom index and routine listing for any comment about this, and was suprised that NOWHERE was anything written about how to adjust the display contrast. As I get older I am finding it manditory that I have bright light to get good contrast with this display.

I've tried to access this forums archives on this subject, but I got no response from any archive. Does anyone out there Know how to adjust the display contrast? I have the knowledge and tools for synthetic functions if this is required.


>Does anyone out there Know how to adjust the display contrast?


I don't think there is any way to change the contrast of the HP-41 display, short of altering the circuitry inside -- and even that may be impossible. I just don't know electronics or the innards of the machine.

Are you sure your calculator's LCD isn't defective? I have a 41CX that has plenty of contrast.



No, the display is not faulty. It looks exactly like any other Halfnut. But, put it next to a Fullnut and you see one of the big advantages (perhaps the ONLY advantage) of the Fullnut. Even when I was in my 30's with young eyes I prefered the wider segments.

There was a key sequence that adjusted the contrast - I think it was a register entry that altered it. Don


Hi Donald,

I'm sorry but I bring no good news for you, Fullnut's display contrast is by no means adjustable.

I've read somewhere in the archives that Machine-Code Hepax module has a so called "CTRST" function wich allows contrast adjustment in the Halfnut display ranged from "0" to "15".

May be your best option is to get an Halfnut, (thats the easy part as there are always some of'em at eBay, not cheap though)... and, this is the hard part, find an Hepax module and manage to get it! The last (and only) one I saw rises above $600...

Anyhow, I wish you best of the lucks.


Isn't there someone out there that can write this code and put it on an eprom for the rest of us to use? Maybe we could start a custom rom and start discussing all the items we want on this rom. Anyone interested?


Hi again,

Such a module require a *really* weird chip, namely a 10 bit word, 4 kiloword, serial-ized clocking interface memory chip... No, you can't step into Radio Shack and ask the clerk... well, you can, of course, but he more likely stare at you as a if Terminator have entered a GunShop... ;-)

There are Ram/Rom box out there (MLDL's), they are half a printer in size (82143) and you can place a couple of EPROM's (2732 and 2716) to simulate any module you have the code of, anyhow you need an EPROM programmer or somebody who can do the job for you, of course portability goes way out.

Again sorry for the bad news... :-(



If you would like to relive the experience of a community developed custom ROM, and if you want to get to the bottom of whether the display contrast can be changed, get the PPC CD-ROMs that Jake Schwartz provides. They contain a tremendous amount of information and history about many HP calculators and especially the 41. Read all about it here:



... I have no means to do it by myself. I thought about PIC processors, because they are easy to find and fairly documented. One of our contributors was testing AtMel controlers, but as I lost about all my e-mails, I cannot find any reference to his name. At least four guys were involved in this subject, being Meindert and Vassilis two of the ones I remember the names. I hope the others understand this is some sort of "senior moment", and I'd like to know who else is dealing with.

I am sure current controlers have enough resources so they can be used to read data (even have this data internally recorded, as many of them have plenty OTP ROM) and send it as a 10-bit "stream" word, keeping synchronous activity with clock and control signals available in the HP41's I/O ports.

I have no means to do it, and my knowledge about HP41's machine language is "zero". But I learn fast... even being a 41 Y.O. "guy".

Luiz C. Vieira - Brazil


I am still working hard on my MLDL2000, and there is some progress. I could not spend as much time as I wanted lately but I am now back on the track again. There will be an interface to the outside world, based on a very simple JTAG-like synchronous protocol. This would support a parallel interface to a PC by bit banging the PC's parallel port. I am already investigating USB controllers which are almost single chip solutions between USB and a number of parallle bits, and that would give the MLDL2000 a nice USB interface. Please note that software has to be written as well, both on the PC and HP41 side. The interface is primarily intended to be able to load and save the ROM and RAM in the MLDL2000.

The MLDL2000 would allow anyone to keep all his/her favourite ROMS in one place, and add functionality in machine code, including the control of contrast.

The (almost, I will try to update later today) latest specifications and VHDL code are available at www.kuipers.to/hp41. Note that this is all work in progress.

As for the status of the MLDL2000: I am currently implementing the WROM instruction, but there is a nastly little bug in the state machine that I am trying to resolve. When that is done it is time to do the PC interface.

There are some questions that I would like to ask the forum:

- Is anyome familiar with the HP41 DATA line and how is works when a module supplies data?

- Is there more specific information available on the extra HEPAX instructions? I know what they do but wants some more specifics. It will be very difficult to implement though...

- Bank switching: Is there a complete overview of all the particulars?


I have an ADVANCED HEPAX (i.e. 16K RAM with the ROM built in). It does have the contrast function, and could be copied (programmed) as M-Code in another device. I believe that some HP-41 ROM Modules were implemented with UV EPROM's. In theory these module cases could be cracked open, UV erased, and then reprogrammed with your favorite functions (M-Code) and Programs (User Code). I believe there is one user in our community that has the required hardware, but has not had time to set it all up and verify its operation.


The CHHU Chronicle V2N4 Page 9 has an article regarding the Halfnut and this article does have the m-code for the contrast function. Also there is information on other m-code instructions as well.


Hi everybody,

It seems that the customized EPROM modules issue still have a lot of interest in this forum (easy to understand when looking at e-Bay prices :-)...

Let me see if I've understood the previous message...

- Are there EPROM chips (those UV erasable) that fit into a standar HP-41 module case??

- If the answer is YES, are there any clue about which modules could carry this (almost magic) chip?

- AFAIK, ROM modules are 10bit/word and RAM modules are 8bit/word... Anyhow, some modules share machine-code and user code programs in the same device, Does someone knows how they overcome the word lenght difference?

- Is it possible to place a small Lithium battery inside a, said, Quad-Ram module to turn it into a pseudo ROM?

... Well, I'm abusing I know, but those questions have been prowling into my head for long, since I'm a "harware guy" (those with the soldering iron and tweezers), sometimes messing my brain up, and you seem to be the only good answers source.

Thanks in advance and best whishes from the Canary Islands.

P.S. Please forgive my grammar "crimes".


I want to thank everyone who has contributed to this thread. I will continue to monitor this to see if anything further developes. Don

Possibly Related Threads...
Thread Author Replies Views Last Post
  Can the display for a HP32Sii be replaced with the display from another model? Bruce Larrabee 8 1,156 10-10-2012, 10:05 PM
Last Post: Luiz C. Vieira (Brazil)
  Contrast in HP42s flush display , another one.... aurelio 0 340 09-01-2012, 08:47 AM
Last Post: aurelio
  HP 41CX unusual display Bill Kleinsorge (Austin, TX) 5 670 01-20-2012, 10:37 AM
Last Post: Fouad M. Kaadou
  HP 27S - Adjust screen contrast? M Tulloch 2 454 07-13-2010, 02:09 PM
Last Post: M Tulloch
  HP-42S work-around for sticking to low contrast LCD problem Lyuka 2 472 02-01-2010, 01:25 PM
Last Post: DavidShenk
  An HP-42S fix success, and a contrast problem DavidShenk 19 1,637 12-05-2009, 10:17 PM
Last Post: Lyuka
  16 segment Led display would mimic a HP-41 display? Giuseppe Marullo 3 645 11-12-2009, 08:08 PM
Last Post: db (martinez, ca.)
  Changing display contrast on HP 27s MitchT 7 767 01-04-2008, 10:51 AM
Last Post: MitchT
  Power consumption versus contrast? Dave Shaffer (Arizona) 3 502 10-23-2007, 01:43 PM
Last Post: Garth Wilson
  HP48G screen contrast adjustment. Dia C. Tran 1 348 08-06-2007, 09:33 AM
Last Post: Gerson W. Barbosa

Forum Jump: