Too many - after starting on an HP-97 in 1979 I was part of the PC/Internet vanguard. I got hold of a time capsule HP-41CX 3 years ago and have started playing with both vintage and new programmable calculators. There is an iPhone app version called i41cx+ which is an excellent exact emulator of the HP-41CX plus printer, card reader and all known software modules and no I didn't write it or sell it. It just amazes me today (I'm 68 for a few more days) how much bloatware we put up with from software and operating systems - I've just loaded RISC OS onto a Raspberry Pi (some say it was the first GUI see BBC Micro, Acorn Atom PCs) and I think it's 12.8MB (yes MB) for an Operating System. I used SuperCalc in early 80s and it was something like 760KBs and Wordstar wasn't that big either - compared to the size of Excell and Word in their GBs today. However the first commercial PC the company I worked for in 1983 that we bought was a Sirius 1 and cost £2,400 (look up its spec, laughable today) and a year later we bought an external 10MB hard drive for £3,000!

You can run the TI-57 internally with a 9v battery but NOT the TI-59, you'll fry it. I used a 3 AAA battery pack used for a Tomy baby alarm as a replacement pack. If you have an Elektronika MK-61 calculator, it's power supply will run the TI-59.
The major difference is the 59 has a built in card reader.
Denny

Hi I'm a very long in the tooth IT guy and Session Guitarist, now retired. I started in IT in 1979 writing small business applications on an HP-97 ending up in charge of a worldwide network running on servers and mainframes. A few years ago I came across the HPCC after buying a time capsule unused HP-41CX plus printer and card reader. Since then I've concentrated on programming and repairing vintage programmable calculators and recently I was sent a free TI-59 from Israel that had belonged to a scientist who had worked at Los Alamos. I quickly repaired the battery pack and it's the first 59 that the card reader is working.
In the last 2 weeks I've repaired a Commodore PR 100 (new battery pack - the internal 3 AAA pack for a Tomy baby monitor is ideal) although I can't find a printed manual or 6v DC ADAPTOR. I've also bought another time capsule, an unused Sharp PC-1251 pocket compute with its CE-125 base unit with tape micro cassette and printer.