The HP-41 is the calculator that became a system. HP launched the 41C in July 1979, the first calculator of its kind with an alphanumeric display, so programs could prompt you in words. Four ports on its top edge took memory, application pacs, a card reader, a printer, a barcode wand and later the HP-IL loop. BYTE called it the most versatile machine ever in 1980, and HP-41Cs flew on early Space Shuttle missions. Three models were made, and the differences come down to what's built in.
What all three share
- The display: 12 alphanumeric characters on an LCD, each made of 14 segments plus three for punctuation. It isn't dot-matrix, whatever some listings say.
- The keyboard: RPN, one gold shift key and an ALPHA key for letters.
- Power: four N cells, or HP's 82120A rechargeable NiCd pack. HP rated a set of alkalines at up to six months or more of normal use.
- Continuous Memory: programs, data, flags and key assignments survive switching off.
- Four ports on the top edge, and HP's warning to switch off before you plug anything in.
The HP-41C (1979)
The 41C, code name Coconut, has 63 registers of main memory, about 441 program steps if you use it all for programs. That filled up fast, so owners plugged in memory modules: up to four HP 82106A modules or one 82170A Quad Memory Module, for 319 registers. On a 41C with memory modules, application modules have to sit in higher-numbered ports than the memory. It was made until January 1985.
The HP-41CV (1980)
The 41CV, code name Silverbird, arrived in December 1980 with the Quad Memory built in. That's 319 registers and all four ports free. V is the Roman numeral five, for five times the 41C's memory. HP made it until November 1990, so there are plenty about.
The HP-41CX (1983)
The 41CX, code name Honeynut, was the last 41, made from October 1983 to November 1990. HP took the 41CV and built in two modules most owners were buying anyway: the Time Module and the 82180A Extended Functions/Memory module. So it has a clock, calendar, alarms and a stopwatch, 124 registers of extended memory and a text editor on top of the 319 main registers, with all four ports still free. It has over 200 built-in functions and runs every 41C program and module unchanged.
How to tell them apart
- Memory at start-up: after a memory reset, a 41C offers 17 data registers and a 41CV offers 273. That's in HP's operating manual, and it's the quickest test.
- Built-in clock: a 41CX runs CLOCK and EMDIR with no module plugged in. A 41C or CV can't.
- The label: HP's serial label sits on the bottom near the ports. HP's service manual gives the rule: four digits for the year (counted from 1960) and week, then a letter for the country, A for the USA, B for Brazil, Q for the UK and S for Singapore. The manual notes the week code runs eight weeks ahead.
There's also a later internal build, which collectors call the Halfnut. HP's service manual says you can spot one without opening the case by a black frame under the display glass and slightly different character shapes.
The card reader isn't built in
A lot of listings get this wrong. No HP-41 has a card reader inside. The HP 82104A card reader is a separate unit that plugs into port 4, runs from the calculator's batteries and reads HP-67 and HP-97 cards too. HP's handbook says dirty heads or dirty cards cause bad reads, and each pack of cards came with a cleaning card. A listing with a reader is selling two things, so check it separately: ask whether it has read and written a card recently.
The modules worth knowing
- Math Pac (00041-15003): matrices, roots, integration, differential equations and complex numbers, from 1979.
- Financial Decisions Pac (00041-15004): compound interest, IRR, NPV, amortization, depreciation and bonds.
- Time Module (82182A): clock, calendar, alarms and stopwatch. Built into the 41CX.
- 82180A Extended Functions/Memory, marked X FUNCTIONS: string and file functions with 127 registers of extended memory. Built into the 41CX.
- 82160A HP-IL module: connects the 41 to printers, cassette drives and other devices on HP's Interface Loop.
HP's application pacs carry 00041-150xx product numbers, while system modules like the Time Module carry 821xxA numbers. HP never made a Math Pac II for the 41, and there was only ever one Extended Functions module, the 82180A. If a CX listing bundles a Time Module or an X Functions module, you're paying for something it already has.
What goes wrong
Batteries. The Clonix project, which builds modern 41 modules, calls corrosion from leaked batteries by far the worst enemy of the HP-41, because it gets into the flexible circuit that links the keyboard, the battery contacts and the ports. HP's own service categories include battery-corrosion damage. Ask for a photo of the battery compartment with the holder out before anything else.
Contacts come next. HP's service manual traces lockups, changed memory and modules that misbehave to poor contacts. If a 41 shows MEMORY LOST every time it powers up, suspect the batteries or the contacts. If you use the 82120A pack, HP warns not to charge a fully flat pack inside the calculator, since that can lock up the keyboard.
Clones and conversions
SwissMicros makes the DM41, DM41L and DM41X, which run 41CX software in modern hardware. The Systemyde 41CL is different: a new processor board fitted inside a real HP-41 case, with 41CX features, a large extended memory and hundreds of module images. From the outside it looks like an original. Clonix and NoV modules hold copies of many ROMs inside real HP module shells. None of these is a bad thing to own, but each should be listed as what it is.
Which one to buy
Buy a 41CX if you want everything built in and four free ports. Buy a 41CV if you'd rather choose your own modules. A 41C makes sense mainly as a collector's piece or when it comes with memory modules. Whichever you pick, the battery compartment decides whether it's a good buy. Our condition checklist covers the rest, and the HP-42S is the later pocket HP that runs 41 programs.
