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7 The Mockingboard
Thomas Harte edited this page 2024-02-15 09:00:16 -05:00
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The Mockingboards are a series of audio expansions for the Apple II, the most common of which provides stereo sound from a combination of two 6522s, connected to two AY-3-8910s. Some configurations also include SSI-263 speech chips.

AY-3-8910s

Each AY is connected to its 6522:

  • port A is connected to the AY's data pins;
  • bit 0 of port B is connected to the AY's BC1;
  • bit 1 of port B is connected to the AY's BDIR;
  • BC2 is connected to +5v; and
  • bit 2 of port B is connected to the AY's reset.

So, logically:

  • BC1 is active if bit 0 is set;
  • BDIR is active if bit 1 is set;
  • BC2 is always active;
  • reset will occur if bit 2 is reset.

The three channels from each AY are mixed together with equal volume.

Effective commands

bits 02 Command
03 reset
4 none
5 read from AY
6 write to AY
7 set register address

6522s

The 6522s are exposed to the processor's bus:

  • the four register select lines are connected to address lines 0 to 3; and
  • the reset and data lines connect as expected.

The 6522 for the left channel:

  • is selected when address line 7 is low; and
  • its interrupt output is connected to the processor's IRQ input.

The 6522 for the right channel:

  • is selected when address line 7 is high; and
  • its interrupt output is connected to the processor's NMI input.

Both also require the board's IOSEL to be active — they appear only in the ordinary IO area for their board.

SSI-263s

The SSI-263s are more directly connected to the processor's bus.

Both SSIs:

  • connect their data bus directly to the processor's data bus;
  • connect their three register select lines to the low three bits of the address bus;
  • use IOSELECT as a prerequisite for chip selection; and
  • connect their A/R output (i.e. the signal that "may be used as an interrupt request for new phoneme data") to the corresponding 6522's CA1.

The left channel SSI-263:

  • is selected when A5 is high.

The right channel SSI-263:

  • is selected when A6 is high.

Clock Rate

All chips are clocked according to the Apple card's phase 1 clock input. So they are clocked at the same rate as the CPU.