Hewlett Packard HP-35S Checksums Now Work! Well, Sort Of.

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Summary: Checksums work on newer HP-35S calculators when order of entry is maintained.

Checksum and Program Entry Background

2/28/2016 - A checksum is a small code that is unique for a piece of information. If two pieces of information have the same checksum then they can be presumed to be identical. For example, if two calculators return the same checksum for a program then we can assume that the program is the same in both.

Checksums are frequently used in electronic devices to validate data. And they are of tremendous importance in programmable calculators that require manual program entry such as HP-33S and HP-35S. These calculators are frequently used for fields like surveying which use complex programs that may require thousands of key strokes to enter. Each key stroke presents an opportunity for error.

Unfortunately, HP-35S has not had working checksums since its introduction. The checksum calculation for identical programs on different calculators returned different values which makes them useless for establishing correctness of entry.

For several years we've offered a program entry service for HP-33S and HP-35S calculators, especially for D'Zign Surveying Programs. The service takes care of tedious program entry for our customers.

Needless to say, lack of checksums has led to some unnecessary pain and unsatisfied customers. Although we try to be clear about entry service conditions and our entry isn't more likely to introduce errors than self-entry, customers unfamiliar with manual program entry and used to flawless electronic transfers sometimes still feel cheated when they have to fix entry errors.

Every once in a while we'd check checksums of two calculators to see if the problem has been fixed. The answer appears to finally be yes, with some caveats.

HP-35S Checksums Are Fixed, With Caveats:

The main caveat is that programs must be entered in order. Checksums will not match if entry was performed in different order on different calculators. Entry does not need to be performed at once and you can stop in the middle to verify checksums of programs entered thus far.

We do not know when exactly this change occurred. All calculators above serial number CNA5190K8B (manufactured after April, 2015) appear to have this new functionality. This is almost certainly not the earliest serial number; if you know of an earlier one, please let us know and we'll update this information.

Please note that we've only verified this on brand new or reset calculators. We have no idea what exactly affects the checksum calculation. If order of entry does, then variables, settings, program runs, etc. also might.

Some entry errors seem to affect accuracy of checksums even after they are corrected. We've seen situations where checksums on a program would not match, but the program appeared to work and no error could be found despite several careful checks. We haven't found a reliable pattern yet, but these situations seem to only occur after unusual entry errors, such as those that relabel programs.

Please let us know if you have any further insight into what does, and doesn't, prevent this feature from working.

Even in this limited implementation, this is a tremendous improvement that allows program entry to be confirmed correct in many instances. To help customers who self-enter Surveying Solutions for the HP-35S we are also posting an updated table of program sizes and checksums.