The Museum of HP Calculators

HP Forum Archive 15

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HP-85 bus and clock signals
Message #1 Posted by Roberto Bazzano on 16 Jan 2006, 10:28 a.m.

Hello. I need to replace a defective system rom on a HP-85A. Due to the fact that's almost impossible to find, I think to make a small board with a microcontroller to replace it. From the service manual, I understood how the bus and LMA,RD,WR lines work. What I don't know is how to relate them to the four system clocks. I mean, how can the rom know when on the bus there is address LSB and MSB? What does the rom do during negative transition of 4 clock signals?

Probably it's the same method used for external roms (with the addition of i/o rom select) but I dind't find any reference.

Could anyone help me?

Thank you very much. Roberto

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