Details
The PBI is implemented as a rectangular 50-pin edge connector on the back of XL machines. The pins include all 16 address lines (A0 through A15) and 8 data lines (D0 through D7). Other pins include various electrical supplies, clock signals, interrupts, memory strobes, and even an analog audio input.
As many of these pins were already implemented as part of the already-existing 30-pin ROM cartridge slots on Atari machines, the XE series re-implemented the PBI to include only those pins that the PBI had but the cartridge slot didn't. The resulting connector contained only 14 pins, mostly the address pins A13 though A15, and the various sound and signal pins of the PBI. The ECI slot was placed beside the cartridge slot, allowing a single connector to be inserted into both connectors and re-create the functionality of the entire PBI.
Read more about this topic: Parallel Bus Interface
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