2A 3 Floating Bus Test
Author: Hikaru
| Version | Release date | Links | Download |
|---|---|---|---|
| v1.2 | 2017-09-15 | Forum post |
tap
(hosted) -
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| v1.2 | 2017-09-13 | Forum post |
tap
(hosted) -
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| 48K | 128K | +3 | ULA bus |
|---|---|---|---|
| 14338 | 14364 | 14367 | (0x4000) |
| 14339 | 14365 | 14368 | (0x5800) |
| 14340 | 14366 | 14369 | (0x4001) |
| 14341 | 14367 | 14370 | (0x5801) |
| 14342 | 14368 | 14371 | IDLE |
| 14343 | 14369 | 14372 | IDLE |
| 14344 | 14370 | 14373 | IDLE |
| 14345 | 14371 | 14374 | IDLE |
During idle cycles the 48K and 128K ULAs will return FFh for port reads from unattached ports whereas the +2A and +3 ASIC, when memory paging is enabled, will return the most recently cached value from a display byte (see the above table) or a byte read from or written to contended memory by the Z80, but with bit 0 set in all cases, assuming no Centronics printer is connected. The +2A/+3 floating bytes will appear on all ports following the Centronics port decoding (0000 ---- ---- --0-). With paging disabled the +2A/+3 will always return FFh.
All timings are relative to the ULA/ASIC asserting the
Tested by Ricardo Martínez Cantero
Published under the terms of the GNU Free Documentation License
