Re: HP 9815A or 9815S? Message #3 Posted by Tony Duell on 29 Nov 2004, 3:32 p.m., in response to message #1 by Ken
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
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