| ADR | HEXADR | NAME | Description | OS |
|---|---|---|---|---|
| 54019 | $D303 | PBCTL | Port B Control | only A (400/800) |
| Bit | Function | Description |
|---|---|---|
| 7 | PA7 | Interrupt status of SIO |
| 6 | PA6 | always 0 |
| 5 | PA5 | always 1 |
| 4 | PA4 | always 1 |
| 3 | PA3 | Status of command line of SIO |
| 2 | PA2 | 1=use Port B for data input/output, 0=define data direction, see below |
| 1 | PA1 | always 0 |
| 0 | PA0 | Interrupt of PROCEED line on/off, set to 0 by OS |
To define the data direction of PORT B, set Bit 2 of PBCTL to 0. Then write a byte to PORTB, where Bits set to 1 indicate WRITE and Bits set to 0 indicate READ. Normally PORTB is set to %00000000 (=all input).
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