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Thread: Hacking the VT100 keyboard

  1. #11

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    What is the clock rate used by the keyboard UART? Looking at the schematic, it isn't clear. That divided by 128 or 144, depending on which of stop counts are used would be the fastest half cycle you can put out.
    Dwight

  2. #12
    Join Date
    Mar 2006
    Location
    Salt Lake City, UT, USA
    Posts
    295

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    IIRC, the clock is recovered from the signal generated by the main unit. Section 4.4.6.3 Terminal Data Encoding in the VT100 technical manual says:
    The keyboard requires a clock for its operation and is provided with one by the terminal controller side of the interface. To transmit a clock independently of data on the same wire, the terminal side of the interface generates a clock signal within which data is encoded as a pulse width modulation. The terminal circuit produces a 75 percent high pulse width output for the mark state. Data transmission causes the clock output to switch between 75 and 25 percent pulse width (duty cycle).
    Attached is the timing diagram.

    On the CPU side, I am not able to find at the moment the part in the manual where it describes how often the keyboard is issued a command over the bidirectional interface. It states in section 4.4.3 that the key address bit (that starts the key switch scanning counter in the keyboard) is set once per vertical refresh. The same section says:

    The eighth bit, if sent only once, causes the keyboard speaker to click. (Keyclick is defeatable at SET-UP.) If the bit is sent approximately two hundred times in a row, for about a quarter second, it sounds a bell.
    This implies that the command byte is sent approximately 800 times per second.

    clock-diag.jpg

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