HP-85 bus and clock signals



#2

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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