HP-85 ROM select I/O?


Hi folks,

I'm developing a "ROM drawer" for my HP-85 using of-the-shelf 74HC logic chips. In the HP-85 service manual I figured out the auto-increment addressing and the logic to send 16 bit adresses to the ROMs.

I only need to know how to select among several ROMs stored on a FLASH device. What is the I/O address (from $FF00 to $FFFF memory mapped I/O?) to select the right ROM image?

My logic acts as a "decoder" from the HP-85 bus to an ordinary 8 bit data/20 bit address bus and /WE and /RD lines to connect with SRAM and/or FLASH devices.

Many thanks in advance, and best regards!

Nelson Sicuro

Edited: 9 Nov 2005, 8:56 a.m.


It's 177430 in octal or FF18 in hexa.




My HP-85(A) need more RAM and ROMS!! Do you know anything about the RAMdisk? Maybe I use a PIC or other microcontroller to add serial download of the RAM/FLASH contents (USB?).

Best regards,

Nelson Sicuro


No you need an HP85B to have the Electronic Disk. It use the new EMS from the 86-87 line and allow theoritically up to 16MB of extended RAM.

For the HP 85 you can only use this ram as an electronic disk. So the limit is still 32 KB ram (basic). For the rom there is a hard-coded limit for the number of rom. For an HP86-87 it's 15. For an HP85 I don't know.



Do you know anything about the RAMdisk? Maybe I use a PIC or other
microcontroller to add serial download of the RAM/FLASH contents (USB?).

Since you are going to be building the ROM drawer you can use the same logic to emulate the extended memory controller that is provided by the HP85B.

I assume that with that controller in place you will be able to use the HP-85B EMS ROM to create the RAM DISK.

Note that the extra memory will only work as an electronic disk,
you cannot increase the real memory of an 85 (or 85B) beyond 32Kbytes.


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