Zilog Z08470 - Manual (Page 203)

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UM008101-0601 Parallel Input/Output
3. Bidirectional mode: When this signal is active, data from the Port A
output register is gated onto Port A bidirectional data bus. The
positive edge of the strobe acknowledges the receipt of the data.
4. Control mode: The strobe is inhibited internally.
ARDY
Register A Ready (output, active High). The meaning of this signal depends
on the mode of operation selected for Port A as follows:
1. Output mode: This signal goes active to indicate that the Port A
output register has been loaded and the peripheral data bus is stable
and ready for transfer to the peripheral device.
2. Input mode: This signal is active when the Port A input register is
empty and is ready to accept data from the peripheral device.
3. Bidirectional mode: This signal is active when data is available in the
Port A output register for transfer to the peripheral device. In this
mode, data is not placed on the Port A data bus unless ASTB is active.
4. Control mode: This signal is disabled and forced to a Low state.
B7-B0
Port B Bus (bidirectional, tristate). This 8-bit bus is used to transfer data
and/or status or control information between Port B of the PIO and a
peripheral device. The Port B data bus is capable of supplying 1.5 mA @
1.5V to drive Darlington transistors. B0 is the least significant bit of the bus.
BSTB
Port B Strobe Pulse from Peripheral Device (input, active Low). The
meaning of this signal is similar to that of ASTB with the following
exception: In the Port A bidirectional mode, this signal strobes data from
the peripheral device into the Port A input register.