On the HP3500C, Register 0xd0 is a set of bit flags showing the current state of the front panel buttons on the HP3500C. When no buttons are currently pressed, its value is ff. The following bits are clear when the corresponding button is down:
| Bit | Bit value | Button |
|---|---|---|
| 0 | 0x01 | Clear if transparency adapter (TA) is present |
| 2 | 0x04 | Copy |
| 3 | 0x08 | Scan |
| 4 | 0x10 |
Register 0xda records whether these buttons have been pressed since the flag was last written. So, for example, if you press the copy button, bit 2 in 0xda will be set to 1 until the driver resets it to 0. This allows for polling that register without adverse consequences from delay.
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