HP-19C Programmable Calculator

When my HP-42S broke down in 1998, one of my colleagues (also my former grad supervisor) gave me his broken HP-41CX and HP-19C. I described how I repaired the 41CX in another article. Here we'll take a look at the 19C.

The HP-19C was one of a matched pair of high-end programmables in HP's "20 Series" of calculators. The 19C was the printing version of the 29C, which in turn was an improvement over the similar looking 25C. With the exception of the printing functions, the 19C and 29C shared the same keys, although in a different arrangement.

The programming capabilities of the 19C (and 29C) were impressive, with space for 98 fully merged keystrokes, and 30 memory registers. Although these calculators lacked a card reader for permanent storage, both had continueous memory that would retain the most recently entered program and 16 of the 30 registers even when the calculator was turned off (a feature taken for granted today).

This calculator, in addition to offering a full suite of scientific and statistical functions, had very powerful programming constructs. There were eight different conditional tests (four x versus y and four x versus 0), increment and decrement instructions, three levels of subroutines, and indirect addressing of both registers and branch targets. Branching was done using labels, but indirect branching also supported relative jumps. Program editing was made easy by automatic insertion, and a DEL key.

At the time that this calculator was new, I was thirteen years old and had never heard of HP calculators. Instead, I had a Commodore PR100 with 72 unmerged program steps and 10 memories, which falls far short of what the 19C can do. Of course, the Commodore cost far less as well, and I spent hundreds of enjoyable hours writing programs for it.

	The back label contains a list of useful conversion factors.

Now, thirty years later, the HP-19C is the calculator I use most often. It sits on my desk at work, and gets used several times a day for quick calculations. I've also written an equation solver program for it, which is reproduced at the end of this article.

Repairs

As I mentioned in the introduction, my HP-19C required some repairs when I first received it. It needed a new battery pack, the keyboard had become unplugged from the main CPU board, and the tabs on the battery cover were damaged (almost broken off).

I fashioned a new battery pack out of 1100mAh Sanyo NiCd cells. These were state-of-the-art in 1998, and had far more capacity than the original HP battery pack (which I think was 250mAh). Rather than rely on pressure to maintain a connection with the battery contacts, I soldered the battery pack leads directly to the terminals in the calculator.

My home-made 1100mAh 4.8V battery with leads soldered in place.

This calculator was really designed to be disassembled. The back cover comes off after removing a few screws, and the various sub-assemblies inside just plug together. When I received the calculator, the keyboard wasn't properly plugged into the main board (the result of an earlier repair attempt), but it was a simple matter to align the pins and put it back together.

The battery compartment cover has small tabs that hold the cover in place. These were almost broken off by someone attempting to force the cover closed. I repaired these by carefully straightening them, and then running a hot soldering iron along the crease to soften the plastic. The tabs are not as strong as they once were, but they are fairly solid. A piece of foam rubber on the opposite end of the cover keeps it securely in place.

What About the Printer?

The printer on my HP-19C sort of works, except for two problems: two of the print element dots are cracked, and I can't get any paper for it anyway. I cut some appropriately narrow strips of paper from a roll of thermal fax paper to test the printer, and noticed two rows of dots missing. Upon disassembling the printer, hoping to find a loose connection, I discovered cracks in two of the print head's heating elements. Testing with an Ohm-meter confirmed that these were open-circuit. But even without the printer, this makes a nice desktop scientific calculator, and it's the one I keep on my desk at the office.

An Equation Solving Program

One feature of the HP-42S that I missed was the equation solver, so I set out to write a simple one for the 19C. I submitted it to the Museum of HP Calculators, and it now appears in their software library. I've also reproduced it here.

The HP-42S has a solver where you can provide a program for an equation of n variables, fix any n-1 of these variables, and solve for the remaining one, so I wanted mine to have the same flexibility.

My solver uses the secant method, in which the two most recent guesses are used to define a line. The point where the line intercepts the x-axis is used as the next guess. When two consecutive guesses are the same, the solution has been found. I'm sure this solver is not as good as the one in the HP-42S, but it works sufficiently well for my purposes. It can get into an infinite loop on periodic functions, like sin(x).

Usage

Using the solver is simple. First, rearrange your equation so all the terms are on the left hand side. In other words, rewrite it in the form f(a₁,...,a_n) = 0.

Next, enter the left hand side as a subroutine with label 9. The variables are represented by the like-numbered registers (i.e. a₁ is in register 1, and so on).

To solve for any one variable, store values for all the other variables in the appropriate registers, enter two initial guesses for the variable you wish to solve for, enter the variable number, and press GSB 0.

See the sample problem for more details.

Program Listing

Line	Instruction	Comments
001♦	LBL 0	Main entry point
002	STO 0	Store index of variable to solve for
003	R↓
004	STO .2	Store second guess
005	R↓
006	STO .1	Store first guess
007	STO i	Compute f1 = f(R1,..,Ri1,..,Rn)
008	GSB 9
009	STO .0
010	RCL .2	Compute f2 = f(R1,..,Ri2,..,Rn)
011	STO i
012♦	LBL 1
013	GSB 9
014	STO .2
015	RCL .1	Compute new Ri2 = (Ri1 f2 - Ri2 f1) / (f2 - f1)
016	×
017	RCL i
018	STO .1	Move old Ri2 to Ri1 while we're here
019	RCL .0
020	×
021	−
022	RCL .0
023	RCL .2
024	STO .0	Move old f2 to f1 while we're here
025	x↔y
026	−
027	÷
028	STO i	Save new value for Ri2
029	RCL .1	Compare to previous guess
030	x≠y?	Keep going until they're the same
031	GTO 1
032	RTN

Registers

Register	Use
0	Index i of variable to solve for
1-9	Variables to solve for (up to 9)
.0	Previous value for f(R1,..,Rn)
.1	Previous value of Ri
.2	Second guess during initialization. Current value for f(R1,..,Rn) during main loop

Sample Problem

The net resistance, R₃, of two parallel resistors of resistance R₁ and R₂ is given by:

This can be rewritten in the form f(R₁,R₂,R₃) = 0 as follows:

The following subroutine implements this equation:

Line	Instruction	Comments
033♦	LBL 9	Solver uses subroutine with label 9
034	RCL 1
035	RCL 2
036	×
037	RCL 1
038	RCL 2
039	+
040	÷
041	RCL 3
042	−
043	RTN

What is the resistance of a 5kΩ and 10kΩ resistor in parallel?

Description	Keystrokes	Display
Store 5 in R1	5 STO 1	5.0000
Store 10 in R2	10 STO 2	10.0000
First guess is 3	3 ENTER	3.0000
Second guess is 4	4 ENTER	4.0000
Solve for R3	3 GSB 0	3.3333

The answer is 3.3333kΩ. What resistance is needed in parallel with a 10kΩ resistor to give a 2kΩ parallel resistance?

Description	Keystrokes	Display
Store 2 in R3 (R2 is still 10 from the previous problem)	2 STO 3	2.0000
First guess is 3	3 ENTER	3.0000
Second guess is 4	4 ENTER	4.0000
Solve for R1	1 GSB 0	2.5000

The answer is 2.5kΩ. If the program cannot find a solution, it will eventually end up dividing by zero, which will display Error. For example, what resistance is needed in parallel with a 10kΩ resistor to give a 12kΩ parallel resistance?

Description	Keystrokes	Display
Store 2 in R3 (R2 is still 10 from the previous problem)	12 STO 3	2.0000
First guess is 3	3 ENTER	3.0000
Second guess is 4	4 ENTER	4.0000
Solve for R1	1 GSB 0	Error

It's not possible to put something in parallel with a 10kΩ resistor and end up with a higher resistance.

 

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Last updated Friday July 23, 2010.

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