HP 9815A or 9815S?


I have a HP 9815A or S model desktop calculator, but I am trying to find out some information. The keyboard is marked 9815A, but under the printer cover there is an added label that says 9815S and two other lables that say "option 001" and "option 002". Is this a 9815A that was upgraded to the "s" configuration? It seems the primary difference is the number of program steps. How do you get the calculator to list the total number of program steps available? Also, is this a thermal printer similar to the hp 97 or a regular printer? The serial number starts with 16 indicating it was built in 1796 and the ship date marked on the label is 1978. This seems to pre-date the S model, but when I switch to program, 3720 is listed on the right of the display (I assume this is the number of program steps) and it counts down when steps are entered. This seems to imply the 3800 steps available only in the S model. The A model could have upgrades to increase the number of steps to 2008. It does have the two IO ports. I don't know what the options 1 and 2 are?

Edited: 28 Nov 2004, 1:08 p.m.



"...indicating it was built in 1796..."

Are you sure?



As I understand it, the difference between the -A and -S models is the CPU board.

The -A CPU board uses 2111 RAMs, 256 nybles each. There are 6 on the CPU board (for a total of 768 bytes of memory), and you can add a daughterboard with a further 12 chips on it, adding 1536 bytes of memory. This board also uses 2K byte ROM chips -- 7 of them for 14K bytes of firmware.

The -S CPU board uses 2114 RAMs, 1K nybbles each. There are 8 of them, so 4096 bytes of memory. You can't add any more RAM to this board. It also uses a pair of 8K byte ROMs, of which 2K of one of the chips isn't used (so again 14K of firmware).

[Now for something interesting. That 2K is not blank, it looks to be a valid extension ROM for the 9815. If you add a soldered jumper on the board you enable it, and the machine will certainly recognise it. I've not figured out what the functions are yet, though]

I believe one of the OPTions you have is the I/O slot assembly. No idea what the other is. Maybe the extra RAM daughterboard. Maybe your machine started off as a -A, and was later upgraded to a -S by replacing the CPU board -- this is entirely possible to do


I have two hp-9815s calculators. Both of them are labeled 9815A on the keyboard and 9815S under the printer cover. I am the original owner of one of them. It was ordered as a 9815S in 1978. I have found a great deal of technical information on the 9815 on a web site that has many hp patents listed. Many of the patents can be downloaded. The site is www.brouhaha.com/~eric/hpcalc/patents/. There are two patents on the 9815 if I remember correctly. One of them documents the entire design, right down to a flowchart of the program and an assembled listing in MC6800 assembly language. It also includes the complete schematic diagram of each board. You might be able to find the hidden 2K in this documentation. Let me know....

Jim Butch


Yes, I've seen that patent. There are _some_ minor differences between the machine in the patent and the 9815A, and the 9815S, of course, has even more differences. It's still a useful resource....

Anyway, the ROM listing is incomplete (it doesn't contain the maths routines), but there is a description of the extension ROM format, which is how I recognised the extra 2K in the 9815S. But since the patent really describes the 9815A, it doesn't mention this block of code at all.


So it is a 9815S. Can you provide any insight into what Option 001 and Option 002 are?


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