This is a generic I/O chip designed by Motorola and manufactured by Thomson as a second source. It is present in every Thomson machine.
The 6881 exposes 4 registers to the CPU. They are, in this order:
The control registers have the following functions:
This bit is set on transition (low>high or high>low) of the CA1 or CB1 pin. It is cleared by reading the output register.
This works the same as bit 7, but is only used when CA2 or CB2 is configured as an input pin. Otherwise, the pin can't be used as an interrupt source.
When bit 5 is 0, CA2/CB2 is set as an input pin, and can be used for interrupts. In this case, the two other bits have the following meaning:
When bit 5 is 1, CA2/CB2 is set as an output pin.
Setting this bit to 0 maps the data register to “direction configuration”. In this mode, writing to the data register allows configuring each bit as an output or input.
Setting it to 1 maps the output register instead, allowing to read and write the current level of the 8 bits on each port.
In this mode, the PIA can be used to map other peripherals indirectly to the CPU bus. Pins CA2 and CB2 are used as read and write signal lines. This allows communication with slow peripherals, with the following scheme:
Bit 7 is low when no tape drive is plugged, and high when there is one. The monitor loading and saving code checks for this to detect the tape drive.
This bit is set by the keyboard system while a key is pressed. This allows the keyboard handler code to know if it needs to do a full keyboard scan.
These bits are wired to printer port D1-D7. Bit 0 is PB0 (this would have been too simple!)
set to 0 to enable video incrustation mode. Note this needs specific hardware for synchronizing the system clock to the external video.
These bits allows to map some of the expanded memory in TO machines. When no internal RAM is selected (bits are all 1), an external expansion (on the TO9) may be used. This is controlled by bits P2 and P6 of the 6846 PIA.
| P6 | P2 | PB7 | PB6 | PB5 | PB4 | PB3 | Mapped memory at A000-DFFF |
|---|---|---|---|---|---|---|---|
| X | X | 1 | 1 | 1 | X | X | TO7 32K expansion mappings |
| X | X | 1 | 1 | 1 | 1 | 0 | Internal bank 0 |
| X | X | 1 | 1 | 1 | 0 | 1 | Internal bank 1 |
| X | X | X | X | 0 | 1 | 1 | TO9 extra internal RAM mappings |
| X | X | 0 | 0 | 0 | 1 | 1 | Internal bank 2 |
| X | X | 1 | 0 | 0 | 1 | 1 | Internal bank 3 |
| X | X | 0 | 1 | 0 | 1 | 1 | Internal bank 4 |
| X | X | 1 | 1 | 0 | 1 | 1 | Internal bank 5 |
| X | X | 1 | 1 | 1 | 1 | 1 | TO9 external RAM mappings |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | RAMdisk 1 |
| 1 | 0 | 1 | 1 | 1 | 1 | 1 | RAMdisk 2 |
| 0 | 1 | 1 | 1 | 1 | 1 | 1 | RAMdisk 3 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | RAMdisk 4 |
And to add to the fun: instead of simply programming the registers to 0 or 1 using the data port, you must program them as inputs (for 1) or outputs (for 0). The data register is always at 0. the idea is to allow the other bits (0-2 and 7) to be used safely, without risking to enable two RAM banks at the same time.
Seriously, this system is a mess. Use the new banking from the gate array if you can.
This is connected to the video incrustation CLRG signal. The only use is detecting that the signal is available from software. This is useful, because switching the video incrustation on without this would crash the machine.
Active low.
Triggered when the lightpen sensor detects the screen refresh.
Available as an expansion for the TO7, TO7/70, TO9 and MO5, this is built-in for the other machines.
Data port A (x7CC)
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Data port B (x7CD)
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The joystick buttons are wired both to port input pins and to control pins. This makes it possible to trigger interrupts on them, but read the complete joystick state for both joysticks on data ports A and B without having to also test the control ports.
It is not possible to use the DAC and the joysticks (or the mouse) at the same time! This is particularly annoying for the mouse, as one of the quadrature encoded signals will interfere with the DAC. So you have to move the mouse very slowly until it is not interfering anymore. Or, you can unplug the mouse when it is not in use.
The sound output from the DAC can be muted by using the 6846 buzzer output to force it low. Or, the DAC cn be used as a “volume control” for the buzzer, because of the way they are mixed together.