Parallel Bus Interface
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The Parallel Bus Interface, or PBI, is a 50-pin port found on some Atari 8-bit XL computers. It provides unbuffered, direct connection to the system bus lines (address, data, control), running at the same speed as the 6502 CPU. The 600XL and 800XL computers, along with the unreleased 1400XL and 1450XLD had a PBI interface.
The Enhanced Cartridge Interface, or ECI, was a modified version of the PBI designed to be smaller and less expensive to implement. Many of the pins in the PBI were duplicated in the 30-pin cartridge slot, so ECI was limited to only those 14 pins in the PBI that were not in the cartridge slot. Placed side-by-side on the back of the computer, devices plugged into both at the same time to provide the same electrical interface as the PBI. The ECI was found on the 65XE (late production units only), the 130XE and the 800XE.