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The Mockingboard
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.
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.
| bits 0–2 | Command |
|---|---|
| 0–3 | reset |
| 4 | none |
| 5 | read from AY |
| 6 | write to AY |
| 7 | set register address |
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.
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.
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.