HP 86: Strange Error

[Paul Berger (Canada)]

Unfortunately the ROMs used in these machines are not industry standard ROMs, this is why we have things like the PRM-85. The ROMs have address selection logic on the die and in the case of ROMs that are bank selected they have the logic that monitors the bank select address and recognizes its ROM number. The internal ROM like the printer and mass storage ROM fitted in the 86A would be the same as ROM as the one you would plug into a ROM card except that it is in a DIP package instead of being wire bonded to a small circuit board inside a plastic module. Cards like the PRM-85 put all this address selection and bank selection logic into a CPLD so that industry standard ROMs can be used to replace the HP custom ROM. It would be possible to make a single image module like this to replace one of the internal ROMs, however if one of them is defective, it might be just as easy to put the image onto something like a PRM-85. One of the OS ROMs is also bank selected but the remaining 3 are at fixed addresses in the memory map, however due to the unique way address and data transfers take place with the Capricorn processor you could not drop an industry standard ROM in to replace them either.