Differences between the HP 15c CE and the original HP-15C

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Differences between the HP 15c CE and the original HP-15C
The HP 15c CE Owner's Handbook describes all the major differences between the HP 15c CE and the original HP-15C. However, there are some additional minor differences, which arise from the complexities of making firmware written 40 years ago communicate across hardware from today. The danger of creating a problem by fixing what are really cosmetic issues was not deemed worth the risk. This is a more comprehensive list of all known differences.

Performance

The CPU of the HP 15c CE is more than 200 times faster than the CPU in the original HP-15C. Due to overhead, real-world performance does not increase by this much.

If a program written for the original HP-15C relies on special timing, for example if the expected run-time of a program gives the user a fixed time to react, then that program will need to be slowed down on the HP 15c CE, possibly by the addition of [PSE] (pause) steps or by making loops repeat more times.

Memory

The HP 15c CE provides 50% more memory than the HP-15C — 672 bytes instead of 448 bytes. This increases the number of allocatable storage registers from 64 to 96.

This means the [g] [MEM] function, which displays the available registers and their allocation, will appear different due to the increased memory on the HP 15c CE. The default allocation shows “19 78 00-0” instead of “19 46 0-0”.

The original HP-15C was limited by memory to a maximum matrix size of 8 × 8. Despite the increased memory of the HP 15c CE, the limit of 8 × 8 remains for matrix inverse, determinant, and division.

Battery Usage

The greater performance means greater battery requirements, too. The HP 15c CE uses two lithium batteries instead of three alkaline or silver-oxide batteries in order to provide sufficient current.

The low-power indicator is designed to give you plenty of warning before the calculator stops functioning. The calculator does this by checking periodically after keystrokes to see whether the batteries will soon need replacement.

To minimize power consumption, the calculator normally operates in a low-power mode, with the processor running at a slow speed. When running user programs (including integration), the calculator switches the processor to full speed, which increases the demand on the batteries.

However, when the low battery indicator comes on, the calculator will only run at the slow speed to avoid overloading the batteries.

Most programs will complete in a second or less, often much less, and appear to be instantaneous. But if your program is designed to run for very long run times (i.e., exceeding 10 seconds), it is possible that before the low battery indicator comes on, you can deplete the batteries to below level that safely maintains memory, so some of your programs and/or data could be lost.

To avoid this, be sure to have new or relatively fresh batteries installed before executing user programs with extended run times. Note that this "difference" exists with similar products from other manufacturers.

Self-Tests

Because of the different hardware platform, the HP 15c CE has a different self-test system than the HP-15C had. This means the self-test operations provided on the original HP-15C no longer give valid results. The new self-test procedures are described in Appendix F of the Owner's Handbook.

Display

The original HP-15C had fixed display contrast. The contrast on the HP 15c CE can be adjusted by following the steps on page 63 of the Owner's Handbook.

The behavior of the flashing "running" display is also slightly different on the HP 15c CE. Sometimes the "running" text will flash at a different speed than on the original HP-15C. It may even not flash at all, for instance, during loops of complicated MATRIX instructions (50 repetitions of 8x8 determinants which takes about 20 seconds to run is one example), but given the processor speed of the HP 15c CE, this is unlikely to be noticed with any user program. It does not affect any computational results.

The HP 15c CE display may blink due to timing differences with the "running" display state and the original HP-15C hardware. One case of this has been reported when executing a program containing combination calculations, but it is suspected it could occur in other circumstances. Usually a blinking display indicates an overflow or underflow and flag 9 is set. The cases where the HP-15c CE display is blinking appear to be a result of the timing differences and not an error in calculations - flag 9 is CLEAR in the example found which means no error was seen. The way to tell if there is an actual error vs. a screen artifact is to press a f or g shift key. If the blinking stops, it was an artifact. If it continues to blink, an error has occurred. No known errors in computations are expected and none have been found as of this post.

Key handling during “temporary displays” is different on the HP 15c CE. Temporary displays are when you press [g] [MEM] to temporarily display the memory allocation, when you press [f] [PREFIX] to temporarily display the full mantissa, or when, in complex mode, you press [f] [(i)] to temporarily display the imaginary component. In these three cases, while the temporary display is up, key presses are accepted on the HP 15c CE rather than ignored as they were on the original HP-15C. If you press a single digit key, the digit you pressed will be displayed and flashing. Pressing any additional key will stop the flashing. Since this behavior is not intended on the original HP-15C, the best way to avoid this difference is not to press a key during a temporary display of course.

Synthetics

"Synthetics" on the HP-15C original fall into what PPC termed NOMAS (Not Manufacturer Supported) and were never core functionality in any way described in the original manual. As mentioned below, they were a "trick" - an unsupported one - and those tricks are not present in the HP-15C CE.