science

Programmable logic array

Programmable logic array is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Programmable logic array rather than just read about it. In short: A programmable logic array (PLA) is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output.

Key takeaways

Programmable logic array belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Programmable logic array to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Programmable logic array from memory before moving on to harder problems.

Reference excerpt

A programmable logic array (PLA) is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output. It has 2N AND gates for N input variables, and for M outputs from the PLA, there should be M OR gates, each with programmable inputs from all of the AND gates. This layout allows for many logic functions to be synthesized in the sum of products canonical forms. PLAs differ from programmable array logic devices (PALs and GALs) in that both the AND and OR gate planes are programmable. PAL has programmable AND gates but fixed OR gates

History In 1970, Texas Instruments developed a mask-programmable IC based on the IBM read-only associative memory or ROAM. This device, the TMS2000, was programmed by altering the metal layer during the production of the IC. The TMS2000 had up to 17 inputs and 18 outputs with 8 JK flip-flops for memory. TI coined the term Programmable Logic Array for this device.

Implementation procedure Preparation in SOP (sum of products) form. Obtain the minimum SOP form to reduce the number of product terms to a minimum. Decide the input connection of the AND matrix for generating the required product term. Then decide the input connections of the OR matrix to generate the sum terms. Decide the connections of the inversion matrix. Program the PLA. PLA block diagram:

Advantages over read-only memory The desired outputs for each combination of inputs could be programmed into a read-only memory, with the inputs being driven by the address bus and the outputs being read out as data. However, that would require a separate memory location for every possible combination of inputs, including combinations that are never supposed to occur, and also duplicating data for "don't care" conditions (for example, logic like "if input A is 1, then, as far as output X is concerned, we don't care what input B is": in a ROM this would have to be written out twice, once for each possible value of B, and as more "don't care" inputs are added, the duplication grows exponentially); therefore, a programmable logic array can often implement a piece of logic using fewer transistors than the equivalent in read-only memory. This is particularly valuable when it is part of a processing chip where transistors are scarce (for example, the original 6502 chip contained a PLA to direct various operations of the processor).

Applications One application of a PLA is to implement the control over a datapath. It defines various states in an instruction set, and produces the next state (by conditional branching). [e.g. if the machine is in state 2, and will go to state 4 if the instruction contains an immediate field; then the PLA should define the actions of the control in state 2, will set the next state to be 4 if the instruction contains an immediate field, and will define the actions of the control in state 4]. Programmable logic arrays should correspond to a state diagram for the system. The earliest Commodore 64 home computers released in 1982 (into early 1983) initially used a programmed Signetics 82S100 PLA, but as the demand increased, MOS Technology / Commodore Semiconductor Group began producing a mask-programmed PLA, which bore part number 906114-01.

See also Field-programmable gate array Gate array Programmable Array Logic

References

External links "Programmable Logic Array (PLA)". cmsc311. University of Maryland. 2003. Archived from the original on 2017-12-14. "PLA (programmable logic array)". Java Applet. University of Hamburg. Archived from the original on 2013-01-15. Retrieved 2010-04-07.

Illustrations

Worked examples

Example 1 — a first encounter with Programmable logic array

Start with the simplest possible case. Write down what Programmable logic array claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Programmable logic array before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Programmable logic array ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Programmable logic array

In research: Programmable logic array appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Programmable logic array in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Programmable logic array is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic design automation, Gate arrays, so understanding it makes those chapters shorter.
In everyday life: Look for Programmable logic array outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Programmable logic array in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Programmable logic array means in your own words.
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Frequently asked questions

What is Programmable logic array in simple terms?

Why does Programmable logic array matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Programmable logic array?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Programmable logic array.