Description: Apple II and IIe: Interface I/O Signal Timing (2/97)
Header: Apple II and IIe: Interface I/O Signal Timing (2/97)

 Article Created: 21 September 1984
 Article Reviewed/Updated: 28 February 1997

 TOPIC -----------------------------------------------------------

 This article describes how the I/O strobe signals on the Apple II peripheral
 connector are handled.

 DISCUSSION ------------------------------------------------------

 The I/O strobe signals on the Apple II peripheral connector are decoded from
 the appropriate address lines and combined with the phase one clock. This is
 to reduce the TTL circuitry required to build a simple I/O port.

                         _______________                 _____
 CLOCK 1          _______|               |_______________|

 I/O SELECT       _______________________                 _____
 I/O STROBE                              |_______________|
 DEVICE SELECT

 A simple 8 bit output port would be a positive edge triggered latch with the
 clock tied to I/O select.

                                                 74LS374
                              __________
                             |          |
        GND -----------------| 1        |
        I/O STROBE ----------|11        |
                             |          |
        D0 ------------------| 3       2|---
        D1 ------------------| 4       5|---
        D2 ------------------| 7       6|---
        D3 ------------------| 8       9|---    8 bits of
        D4 ------------------|13      12|---    Latched TTL
        D5 ------------------|14      15|---    Output
        D6 ------------------|17      16|---
        D7 ------------------|18      19|---
                             |__________|

 Assumming that this interface is plugged into slot 1, any write operation to
 $C090..$C09F will transfer the data to the output lines. This is a very
 simple interface, so any read to $C090..$C09F will transfer random data to
 the output latch and to the Apple.


 Article Change History:
 28 Feb 1997 - Reviewed for technical accuracy, revised formatting.
 Copyright 1984-97, Apple Computer, Inc.
 Keywords: