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800 XL PBI Lines

PBI 800xl

1 reply to this topic

#1 lightman OFFLINE  


    Star Raider

  • 52 posts

Posted Thu Jul 30, 2015 6:03 AM

Can anyone explain why the PBI includes the CAS/RAS lines?  It has the full 16-bit address bus, so these seem superfluous.


As I understand, EXTENB will go high when there is an address on the bus from the CPU and then I'm assuming it uses the full 16-bit address space.


#2 Rybags OFFLINE  



  • 15,470 posts
  • Location:Australia

Posted Thu Jul 30, 2015 9:42 AM

You need Ras/Cas when dealing with dynamic Rams.  Internally these signals are generated either by Freddie (XE) or groups of generic logic ICs (most XL, 400/800).

In the case of generating it by logic ICs that adds a fair amount of complexity, space and price to any external device needing it.


So, it's easier to just have those signals present for Ram expansions.  Note that modern day Ram expansions like the 320XL use static Rams so those signals not required - also, the Ras/Cas isn't provided on the XE ECI.

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