Programmable array logic

From Academic Kids

Programmable array logic (PAL) is a programmable logic device used to implement combinational logic circuits. The PALs were the first programmable logic devices for the commercial market, introduced by Monolithic Memories, Inc. (MMI). (disputed)

The PAL, in comparison to the PLA has a fixed set of OR gates, and thus a fixed number of programmable AND planes. However, the PAL allows reuse of function outputs, and can be then used in another PAL program block. (disputed)

Early PALs were 20-pin DIP (dual inline package) components fabricated in bipolar silicon transistor technology with nichrome programming fuses. The 16L8 and 16R8 were popular members of the product family. The devices have fixed-or, programmable-and-plane arrays of transistor cells to implement 'sum-of-products' binary logic equations for each of the outputs in terms of the inputs and either synchronous or asynchronous feedback from the outputs. Before PALs were introduced digital designers would use SSI (small-scale integration) components, such as 7400 series nand gates and D-flipflops. One PAL device would typically replace dozens of such 'discrete' logic packages, so the SSI business went into decline as the PAL business took off. PALs were used advantageously in many products, such as minicomputers, as documented in the best-selling book "The Soul of a New Machine."

Early PALs were programmed using PALASM language files (converted by a compiler into JEDEC ASCII/hexadecimal files) and a special electronic programming system available from either the manufacturer or a third-party, such as DATAIO. Gang programmers were used when more than just a few parts were needed and for large volumes the manufacturer would fabricate a custom metal mask for manufacturing so electrical programming could be eliminated to reduce cost. PALASM was used to express boolean equations for the outputs pins in a text file which was then converted to the 'fuse map' file for the programming system using a vendor-supplied program; later the option of translation from schematics became common, and later still, 'fuse maps' could be 'synthesized' from an HDL (hardware description language,) such as Verilog.

After MMI succeeded with the 20-pin PAL parts, AMD introduced the 24-pin 22V10 PAL with additional features. After buying out MMI (1987?), AMD spun off a consolidated operation as Vantis, and that business was acquired by Lattice Semiconductor in 1999.


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