HP 28C graphing bug

[Osvaldo Rodriguez]

Hello everyone.
I have been a long time reader of this forum, however, never had the need to post a topic until recently. I have searched the archives and other websites to see if anyone else has observed what I ecountered, however, I could not find any direct reference to it.

I own two HP 28C calculators, I have been using one as my daily calculator for several years as a college chemistry instructor. A few months ago the door gave out as it common with these types, so I took out my other hp 28c. I have been using it without problems, however, I have never really needed to use its graphing capabilities. Until a few days ago a student had a non-chemistry related question. The student needed to find the intervals at which F(x)=sqrt(1728-12*X^2) is defined. I went over how to find the zero's, etc. Then I proceded to try and show her how the graph of the function looked ( looks like a candy cane) and to my surprise my hp 28c could not graph it, it gave me a "non-real result" error. I figured it had to be a flag setting. I went home and played around with the calculator.

I was able to graph it if I create a window under which the function is defined at all points, in other words if I zoom in it will graph it, however, if I zoom out it won't. I took the batteries out and reset the calculator. I tried graphing sqrt(x) and 1/x and it was unable to graph them under default settings. For 1/x I obtained "infinite result" error. I was able to get the calculator to graph 1/x by clearing flag 59. However, I could not figure out a way to graph functions that include radicals.

I also tried the original hp 28c and this problem was not present. I checked versions, and the bug seems to be present in version 1BB and apparently fixed in version 1CC.

In recent years HP has taken a beating on how it has released calculators with some bugs which seemed obious, and there is always mention of the good old days of the HP. Well it seems that they actually released a calculator unable to graph simple functions like sqrt(x) and 1/x right out of the box. Any thoughts on this bug and ways around it other than zooming in.

Osvaldo Rodriguez

[Bruno Férard]

Hello Osvaldo,

I tried to reproduce your issue on my HP28S, but I was lucky enough to have it working normally i.e. no plot and no error out of the range -24*sqrt(3) --> +24*sqrt(3).

May be this is an issue that was fixed before the release of the 28S.
Let's see if others have experienced such issues.

[Massimo Gnerucci (Italy)]

Have a look here and you'll find many more bugs in version 1BB.

Greetings,
Massimo

[Karl Schneider]

Hi, Massimo --

From the link you kindly provided:

http://www.finseth.com/hpdata/hp28c.html

"This machine brought the functionality : memory ratio to heights never before dreamed."

Man, that's the truth! Symbolic algebra and calculus, dynamic stack, matrix storage, all with a measly 2 kB of RAM. No wonder the HP-28S with 32 kB of RAM was released within about a year.

This reminds of the the P/U (productivity-to-usefulness) ratio introduced in the engineering-oriented Dilbert comic strip, to lampoon workers who crank out an endless supply of, shall we say, "junk product".

Still, I like the HP-28C/S (I have two 28C's) for its unit library and conversions. The dedicated alpha keypad is nice, too.

-- KS

[Bruce Horrocks]

Hi Osvaldo,

I had no problems graphing this on my 28C, serial starting 2740A. Can't remember off-hand how to check which ROM version I have.

These are the relevant settings from my calc:

  EQ:   'sqrt(1728-12*X^2)'

  PPAR: { (-50,20) (50,45) X 1 (0,0) }

  RCLF: # 4001FFC40000000

Plotting with these gives me a nice n-shaped curve and no errors.

I've no idea why you got the errors that you did? Maybe the expression was entered incorrectly but that wouldn't explain why you were getting errors after re-starting the calculator. Sorry, but it works here. :-)

[James M. Prange (Michigan)]

To get the ROM version on a 28C:

HEX #A SYSEVAL

or

DEC #10 SYSEVAL

or

OCT #12 SYSEVAL

or

BIN #1010 SYSEVAL

The above will work on a 28S too, but on a 28S you can use a base suffix, and if you do, you don't have to set the base mode. So the following will work on a 28S:

#Ah SYSEVAL

or

#10d SYSEVAL

or

#12o SYSEVAL

or

#1010b SYSEVAL

Regards,
James

[Osvaldo Rodriguez]

Well I used a door from an HP 18C to get my other HP 28C working, and I have been attempting to graph the same functions on both with identical settings.

I came to the conclusion that the HP 28B version 1BB is unable to graph functions if the graphing windows includes areas where the function is not defined. For example it us unable to graph Log (x) under default settings. But if I close the graphing windows so that only positive values of x are shown it can graph it. None of those issues appear on version 1CC.

This makes me wonder a few things.

How did HP release a production model with such graphing deficiencies? The calculator cannot graph the most basic functions such as sqrt(x), Log (x), Ln (x), and 1/x.

Obiously the issue was fixed in version 1CC, so I ask, how many calculators were sold with the defective ROM? From the responses here, it seems that most people have version 1CC.

From information in the museum it says that the HP 28C was replaced within a little over a year with the HP 28S, so my guess is that the HP 28C with version 1BB might only have been for sale a few weeks at most.

Anyone in the museum with version 1BB that can verify the results that I obtained. Or maybe I just dropped the calculator one too many times.

Osvaldo Rodriguez

[Gerson W. Barbosa]

In fact version 1BB returns "Non-real Result" when graphing sqrt(x) and "Infinite Result" when graphing 1/x. Never had any problem with my HP-28S though.

[GE]

Hi all, I just got a non-working 19B without battery door, and a 19BII in the same state.

I tested both with the battery door from my 28C (1BB - will test the aforementioned bugs), and got no response from fresh batteries.

Does anyone have an idea to ge these machines going ? Otherwise, I'd like to take a look inside just as a training for future fixes. Or if you want them, please post here. Are guys at FixThatCalc or other repair services interested in these dead units (no money involved) ?