88-SIO2JP and FDC+ Interaction

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

88-SIO2JP and FDC+ Interaction

Postby toml_12953 » February 22nd, 2019, 10:16 pm

The FDC+ keeps 256 bytes of ROM enabled even if you disable the ROM (Mike, could you redesign the board in a future revision to eliminate this?)
The 88-2SIOJP allows ROM to be turned off after software boots like the turnkey Altair did. Can it only turn off its own ROM? If not can it turn off the last 256 bytes that FDC+ leaves on?
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Re: 88-SIO2JP and FDC+ Interaction

Postby AltairClone » February 22nd, 2019, 11:38 pm

No, the FDC+ does not “keep 256 bytes of ROM enabled even if you disable the ROM.” Read section “2.2 Configuring On-Board RAM” of the FDC+ manual, specifically the last paragraph in that section. There it states that the last 256 bytes of RAM are not accessible. If the ROM is not enabled, those last 256 bytes are not ROM - just empty address space.

Regarding the second question, the 2SIOJP only can only disable its own ROM - it cannot disable ROM present on other cards in the system.

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