PBI, D6xx and D7xx?

[Cybernoid]

Does anyone know what the "external select" and "external decoder output" do on the PBI?

 

Are these mapped to D7xx or d6xx, for that matter what is mapped to d6xx and d7xx? I know there are outputs from the address decoder, but I am not sure what they do and what is there...

 

Thanks,

C

[+warerat]

Does anyone know what the "external select" and "external decoder output" do on the PBI?

 

Are these mapped to D7xx or d6xx, for that matter what is mapped to d6xx and d7xx? I know there are outputs from the address decoder, but I am not sure what they do and what is there...

 

Thanks,

C

 

The $D6XX/$D7XX selects are unconnected from the I/O decoder on the XL/XE, with the exception of the 800 where they are available in memory slot 3. Some PBI devices and others (i.e. MIO) use this space for additional I/O space.

 

/EXTENB (external decoder output) on the XL is an active-low output and is only valid when there is an access to on-board RAM and invalid when there is an access to a ROM space: OS ROM $C000-$CFFF/$D800-$DFFF (if /MPD isn't asserted)/$E000-$FFFF/$5000-$57FF (if self-test bit enabled), $A000-$BFFF (if basic enabled), or $8000-$9FFF/$A000-$BFFF (if external 8K/16K cart inserted). This one is missing from the XE ECI, but it's the same as /CI (CAS inhibit) on the MMU.

 

When the /EXTSEL (external select) input is asserted, the on-board RAM is disabled. This signal is normally held false through a pull-up. The XE ECI connector has /EXTSEL and is directly connected to FREDDIE to inhibit the RAM. Per the PBI spec, you're supposed to assert /EXTSEL only when /EXTENB is true.

 

Hope this helps....