IIC... ...via the parallel port (on older machines)

• A305
• A310
• A410 / A420 / A440
• A3000
• R1x0 unix machines based on A4x0 design
• A540

The reason for this is because the parallel port provided by this hardware is very minimal, using latches, it is not really any advance over that provided in the BBC microcomputer...

Above is the parallel port implemented on the A310. You can clearly see the 74LS374N logic IC used to provide the port.

Here it is as a schematic; based upon the circuit diagram in the Acorn service manual, only simplified...

Thus, the AutoFD connection is not supplied...

We have one output (!STROBE) and two inputs (!ACK and BUSY).

There are two possible ways to fix this, but first...

Leave SDA alone, wire it as decribed to pins 1 and 11. We have !STROBE and BUSY.

1. If it is NOT necessary to read the state of SCL (if we know we're the only master and our wiring is good), then we can connect SCL to one of the data bits and toggle it as necessary.
    
   
2. As above, but we also connect the SCL input to !ACK.

In either case, the driver will need modifications and it will become obviously less efficient due to its use of two different registers for the data and the clock.

A possible alternative could be to wire the outputs on the data pins, and input via BUSY and !ACK (leaving !STROBE unused).

I have not implemented such a thing so a module for this configuration does not exist. If you are interested, let me know what you think and I'll see what I can do...

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