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14209Re: RCA VIP keypad

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  • Tom
    Jul 1, 2013
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      Hi Scott,

      You are correct, with the minor exception that EF3 goes low when a depressed key is detected. This is in response to a hex 62 instruction that initiates the 4-to-16 line decoding, which latches its output so that the EF3 line can be tested. Since the CD4545 4-to-16 decoder is used in the VIP, the output line is active LOW, providing the proper EF3 signal. The CD4514 would provide an active HIGH.

      If you would like some insight to the ROM, Lee Hart has provided us with a disassembled listing of the VIP Monitor ROM. You can access this at 
      Disassembled VIP Monitor ROM in the Files section of the group. This is a text file, so you can open it with any text editor or viewer.

      - Tom

      --- In cosmacelf@yahoogroups.com, "somaspack" wrote:
      >
      > Hi,
      >
      > The manual for the VIP doesn't go into the details of the ROM monitor. I am curious how the keypad functions. It uses a 4515 4-line to 16-line decoder, the inputs are D0-D3, the outputs are connected to the individual hexadecimal switches, the common end of all switches are wired to EF3.
      >
      > So I suspect that when run, the VIP polls each switch in a loop (0-15), and addresses the 4515, places the relevant nibble on the data bus to enable the key output, and if the key is found pressed - EF3 goes high which would then trigger a branch operation to handle the key press.
      >
      > Does this sound correct? Thanks.
      >
      > Scott
      >
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