The Atari Lynx supports cartridge sizes of 129K, 256K, 512K and 1Mb (bank switched). The Lynx cannot address data on the cartridge directly and instead uses two counters to read data. The first counter is for the block number (0 - 255), the second counter is for the position within the block (0 - 4095, depending on cartridge size). Your code must know the exact block size used on a cartridge to be able to read data from it.
Below is the pinout for a typical Atari Lynx Cartridge.
Pins A12 - A19 are connected to the 74HC164 to select the block.
Pins A0 - A10 are connected to the 4040 to select the position within the block.
Pin A11 is missing.
The Card/OE pin is strobed on reads.
The Swcon/WE pin is strobed on writes.
See more Atari Lynx Programming Resources here.
Original information retrieved from RetroIsle.