HP80 TVM sequence?

[brouhaha]

(01-16-2023 12:41 AM)John Garza (3665) Wrote:  It seems the HP80 needs the parameters in a certain sequence to get the right answer sometimes. I can't find anything about that in the manual. But all the examples seem to enter the values from left to right on the keyboard. Sometimes if I mix up the order, it gives the wrong answer. I know later business calcs didn't do that - my 12C doesn't care about the order. Is this some 'hidden rule' about the 80?

Yes. The HP-80 does only support specific sequences, which are those used in the examples in the manual. Which financial keys are pressed matters in the decision of what formula the calculator uses, but the values are simply in the RPN stack. The financial keys don't have associated storage.

The modern five-variable TVM functionality did not exist in HP calculators prior to the HP-92, introduced in 1976, although some of the earlier calculators did not have the severe order limitations of the HP-80, and added dedicated storage registers for the financial variables.

This divides the HP financial calculators into three eras:

* Limited to specific sequences: HP-80, HP-81, HP-70
* Slightly more general, has dedicated storage registers, but not the modern five-variable TVM formula: HP-22, HP-27
* Modern five-variable TVM: HP-92, HP-37E, HP-38E, HP-38C, HP-12C, and later models

By "modern five-variable", I'm referring to the five variables being used in a single general equation. Prior to that, there were five variables, but separate equations were used for four-variable subsets.