HP 15C


My HP 15C recently came up with a capital "C" in the lower right hand portion of the screen and seems to have slowed down the functioning of the unit. If my memory serves me the "C" designated a complex function but I cannot remember how to make the "C" go away. I have had the unit for 20 years and the manuel is long gone.

Any help would be greatly appreciated.



You must clear flag 8. Press the "g" key, then the "5" key, then the "8" key.


Thanks for this Jeff. I have had exactly the same problem. What exactly does flag 8 do? I notice that the trig functions don't work when it is set.


What exactly does flag 8 do? I notice that the trig functions don't work when it is set.

Flag 8, when set, puts the HP-15C in Complex mode, which uses 5 emory registers to create a parallel stack and Last X register for the imaginary parts of complex numbers.

Also, an entire set of mathematical functions operates in the complex domain, with complex-valued inputs and outputs:

+, -, *, /, sin, cos, tan, arcsin, arccos, arctan, sinh, cosh, tanh, arcsinh, arccosh, arctanh, e^x, ln, log, 10^x, 1/x, y^x, sqrt(x), x^2.

abs, ->R, ->P also work a bit differently in Complex mode.

The 15C was probably the first handheld calc to provide this kind of built-in functionality, in 1982.

Edited: 10 Aug 2004, 3:15 a.m.


I notice that the trig functions don't work when it is set

As noted by Karl, setting flag 8 puts the 15C in complex mode. When in complex mode, the trig functions do work. They work purely in radians mode, however, regardless of the setting of the radians flag. So if flag 8 is clear and you are in DEG mode and you enter 0.5 and press sin-1, you will get an answer of 30. If you set flag 8, enter 0.5 and press sin-1, the display will show 0.5236, which is of course 30 degrees in radians mode. Pressing f COS to execute the (i) function will temporarily display the imaginary part of the answer, which is 0. You can also press f -(the minus key) to execute Re<>Im. This swaps the real and imaginary components permanently, unless you swap them back.

The above is covered on page 121 of my edition of the manual.

Possibly Related Threads...
Thread Author Replies Views Last Post
  Photo of my HP 15c | 15c LE DigiGal 2 371 10-12-2011, 12:34 PM
Last Post: DigiGal
  HP 15c LE vs HP-15C dimensions - BOTH ARE HUGE! Joerg Woerner 4 508 10-03-2011, 06:53 AM
Last Post: Jim Johnson
  HP 15C and 15C LE not the same dimensions? Derek 16 1,129 09-30-2011, 05:33 PM
Last Post: Derek
  Original 15C Keyboard Test Works With 15C LE!!! DigiGal 5 616 09-26-2011, 07:33 PM
Last Post: M. Joury
  How would a new HP 15c+ affect 15c market value? megarat 5 495 07-07-2010, 06:48 PM
Last Post: John Stark
  HP 15C: Any cure for an HP 15C with "snappy" keys? Jeff 2 303 09-05-2001, 08:33 PM
Last Post: Jim

Forum Jump: