HP80 TVM sequence?

[John Garza (3665)]

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?

-John

[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.

[Gene]

Been a while, but here are instructions I posted to a webpage...

HP-80 instructions

[John Garza (3665)]

Thanks all,
That sheds more light on it and the history. At least HP chose a simple 'left to right' mnemonic instead of something totally weird. And best of all, it's good to discover my 80 isn't broken! I guess we get a little spoiled using the newer machines, and don't remember it was a process to get where we are.

-John

[Dave Britten]

There are a few interesting details about the HP 80's operation that will perhaps give you a better understanding of how it works:

There are no TVM registers. All computations are performed directly on stack values. (This goes for statistics operations as well!)

Because of this, the TVM keys simply set internal flags corresponding to each variable and enable stack lift to prepare for the next input.

When computing a result, the calculator checks which TVM key flags have been set to choose which equation it needs to use, and then gets the inputs for that equation from the stack registers.

So only two things really matter here: you've put the values onto the stack in the correct order for the equation (usually left-to-right when looking at the TVM keys), and you've pressed the right TVM keys to turn on the internal flags to select that equation, but the order doesn't matter for this. So as long as you're entering the values in the correct left-to-right order, you can press the TVM keys in whichever order you wish (with the exception of the final selection of the variable to solve for at the end) and still get the correct result. Why you would really want to do this is anybody's guess, but you can.