PHY 201 (Blum)1 Some basic electronics and truth tables Some material based on Chapters 3 through 5 of Digital Principles (Tokheim)

PHY 201 (Blum)2 Logic  Digital Electronics In Logic, one refers to Logical statements (propositions which can be true or false). What a computer scientist would represent by a Boolean variable. In Electronics, one refers to inputs which will be high or low.

PHY 201 (Blum)3 Boola Boola! The expression (Booleans) and the rules for combining them (Boolean algebra) are named after George Boole ( ), a British mathematician.

PHY 201 (Blum)4

5 Boolean operators AND: when two or more Boolean expressions are ANDed, both must be true for the combination to be true. OR: when two or more Boolean expressions are ORed, if either one or the other or both are true, then the combination is true. NOT: takes one Boolean expression and yields the opposite of it, true  false and vice versa.

PHY 201 (Blum)6 Representations of Standard Boolean Operators Boolean algebra expression Gate symbol NOT AA´A´ A AND BAB A OR BA+B A XOR B ABAB A NOR B(A+B)´ A NAND B(AB)´

PHY 201 (Blum)7 Our Notation NOT is represented by a prime or an apostrophe. A’ means NOT A OR is represented by a plus sign. A + B means A OR B AND is represented by placing the two variables next to one another. AB means A AND B The notation is like multiplication in regular algebra since if A and B are 1’s or 0’s the only product that gives 1 is when A and B are both 1.

PHY 201 (Blum)8 Other Notations Ā means NOT A A means NOT A AB means A OR B A&B means A AND B Tokheim uses the overbar notation for NOT, but we will use the prime notation because it is easier to type.

PHY 201 (Blum)9 Other vocabulary We will tend to refer to A and B as “inputs.” (Electronics) Another term for them is “Boolean variables.” (Programming) Still another term for them is “propositions.” (Logic) And yet another term for them is “predicates.” (Logic and grammar)

PHY 201 (Blum)10 (AB)’  A’B’ ABAB(AB)’ ABA’B’A’B’ Note that the output is different

PHY 201 (Blum)11 A Truth Table A Truth table lists all possible inputs, that is, all possible values for the propositions. For a given numbers of inputs, this is always the same. Then it lists the output for each possible combination of inputs. This varies from situation to situation.

PHY 201 (Blum)12 The true one Traditionally we take a 1 to represent true and a 0 to represent false. This is just a convention. In addition, we will usually interpret a high voltage as a true and a low voltage as a false.

PHY 201 (Blum)13 Generating Inputs The truth-table inputs consist of all the possible combinations of 0’s and 1’s for that number of inputs. One way to generate the inputs for is to count in binary. For two inputs, the combinations are 00, 01, 10 and 11 (binary for 0, 1, 2 and 3). For three inputs, the combinations are 000, 001, 010, 011, 100, 101, 110 and 111 (binary for 0, 1, 2, 3, 4, 5, 6 and 7). For n inputs there are 2 n combinations (rows in the truth table).

PHY 201 (Blum)14 Expressing truth tables Every truth table can be expressed in terms of the basic Boolean operators AND, OR and NOT operators. The circuits corresponding to those truth tables can be build using AND, OR and NOT gates. The input in each line of a truth table can be expressed in terms of AND’s and NOT’s.

PHY 201 (Blum)15 ABA’B’ ABA’B ABAB’ ABAB Note that these expressions have the property that their truth table output has only one row with a 1.

PHY 201 (Blum)16 In a sense, each line has an expression Input AInput B Expression 00 (NOT A) AND (NOT B) A´B´ 01 (NOT A) AND B A´B 10 A AND (NOT B) AB´ 11 A AND B ABAB

PHY 201 (Blum)17 It’s true; it’s true The following steps will allow you to generate an expression for the output of any truth table. Take the true (1) outputs. Write the expressions for that input line (as shown on the previous slide). Then feed all of those expressions into an OR gate. Sometimes we have multiple outputs (e.g. bit addition had a sum output and a carry output). Then each output is treated separately.

PHY 201 (Blum)18 Example: Majority Rules ABCMajority If two or more of the three inputs are high, then the output is high.

PHY 201 (Blum)19 Row Expressions ABCRow expressions 000A’B’C’ 001A’B’C 010A’BC’ 011A’BC 100AB’C’ 101AB’C 110ABC’ 111ABC The highlighted rows correspond to the high outputs.

PHY 201 (Blum)20 Sum of products Each row is represented by the ANDing of inputs and/or inverses of inputs. E.g. A’BC Recall that ANDing is like Boolean multiplication The overall expression for the truth table is then obtained by ORing the expressions for the individual rows. Recall that ORing is like Boolean addition E.g. A’BC + AB’C + ABC’ + ABC This type of expression is known as a sum of products expression.

PHY 201 (Blum)21 Minterm The terms for the rows have a particular form in which every input (or its inverse) is ANDed together. Such a term is known an a minterm.

Minterms PHY 201 (Blum)22

PHY 201 (Blum)23 Majority rules A´BC + AB´C + ABC´ + ABC ANDs NOTs OR

PHY 201 (Blum)24 Majority rules A´BC + AB´C + ABC´ + ABC ANDs NOTs OR

PHY 201 (Blum)25 Another Example ABCOut

PHY 201 (Blum)26 Another Example (Cont.) A ’ B ’ C ’ + A ’ BC ’ + AB ’ C + ABC The expression one arrives at in this way is known as the sum of products. You take the product (the AND operation) first to represent a given line. Then you sum (the OR operation) together those expressions. It ’ s also called the minterm expression.

PHY 201 (Blum)27 Yet Another Example ABCOut

PHY 201 (Blum)28 Yet Another Example 2 (Cont.) A ’ B ’ C + A ’ BC ’ + A ’ BC + AB ’ C ’ + AB ’ C + ABC ’ + ABC But isn ’ t that just the truth table for A+B+C? There is another way to write the expression for truth tables.

PHY 201 (Blum)29 Another Example (Cont.) ABCOut In this approach, one looks at the 0’s instead of the 1’s.

PHY 201 (Blum)30 Another Example (Cont.) One writes expressions for the lines which are 1 everywhere except the line one is focusing on. Then one ANDs those expressions together. The expression obtained this way is known as the product of sums.

PHY 201 (Blum)31 Expressions ABCExpression 000A + B + C 001A + B + C’ 010A + B’ + C 011A + B’ + C’ 100A’ + B + C 101A’ + B + C’ 110A’ + B’ + C 111A’ + B’ + C’ This is not yet a truth table. It has no outputs.

PHY 201 (Blum)32 Return to Example 1 ABCOut

PHY 201 (Blum)33 Return to Example 1 (Cont.) The product of sums expression is (A+B+C ’ )(A+B ’ +C ’ )(A ’ +B+C)(A ’ +B ’ +C) Each term has all of the inputs (or their inverses) ORed together. Such terms are known as maxterms. Another name for the product of sums expression is the maxterm expression.

Maxterm PHY 201 (Blum)34

PHY 201 (Blum)35 Comparing minterm and maxterm expressions ABCMinterm Expression Maxterm Expression 000A’ B’ C’A + B + C 001A’ B’ CA + B + C’ 010A’ B C’A + B’ + C 011A’ B CA + B’ + C’ 100A B’ C’A’ + B + C 101A B’ CA’ + B + C’ 110A B C’A’ + B’ + C 111A B CA’ + B’ + C’

PHY 201 (Blum)36 Ohm’s Law V = I R, where V is voltage: the amount of energy per charge. I is current: the rate at which charge flows, e.g. how much charge goes by in a second. R is resistance: the “difficulty” a charge encounters as it moves through a part of a circuit.

PHY 201 (Blum)37 Circuit A circuit is a closed path along which charges flow. If there is not a closed path that allows the charge to get back to where it started (without retracing its steps), the circuit is said to be “open” or “broken.” The path doesn’t have to be unique; there may be more than one path.

Open circuit, closed circuit PHY 201 (Blum)38

PHY 201 (Blum)39 An analogy A charge leaving a battery is like you starting the day after a good night’s rest; you are full of energy. Being the kind of person you are, you will expend all of your energy and collapse utterly exhausted into bed at the end of the day; the charge uses up all of its energy in traversing a circuit.

PHY 201 (Blum)40 Analogy (cont.) You look ahead to the tasks of the day and divide your energy accordingly – the more difficult the task, the more of your energy it requires (resistors in series). The tasks are resistors, so more energy (voltage) is used up working through the more difficult tasks (higher resistances). The higher the resistance, the greater the voltage drop (energy used up) across it.

Resistors in series PHY 201 (Blum)41

PHY 202 (Blum)42 Series Two resistors are in series if a charge passing through the first resistor must pass through the second resistor. It has nowhere else to go.

PHY 202 (Blum)43 Resistors in series Each resistor obeys Ohm’s law V 1 = I 1 R 1 and V 2 = I 2 R 2 The current through the resistors is the same I 1 = I 2 = I R1R1 R2R2 I 1  I 2  V1V1 V2V2 ab

PHY 202 (Blum)44 Equivalent resistance (series) The equivalent resistance is the value of a single resistor that can take the place of a combination Has same current and voltage drop as combo V ab = V 1 + V 2 (the voltages add up to the total) V ab = I 1 R 1 + I 2 R 2 V ab = I (R 1 + R 2 ) V ab = I R eq R eq = R 1 + R 2

PHY 202 (Blum)45 Resistors in series Resistors in series add. The equivalent resistance is larger than either individual resistance. If there are more things one has to go through, it will be more difficult.

PHY 202 (Blum)46 Equivalent Resistance

PHY 201 (Blum)47 One charge among many You are just one charge among many. If the task at hand is very difficult (the resistance is high), not many will do it (the current is low); V=IR, if R is big, I must be small. If the task is easy, everyone rushes to do it. V=IR, if R is small, I will be large.

PHY 201 (Blum)48 More energetic If we had more energy, more of us would attempt a given task. V=IR, if V is bigger, I is bigger. If we are all tired out, few of us will perform even the most basic task. V=IR, if V is small, I will be small.

PHY 201 (Blum)49 Given the choice Given the choice between a difficult task and an easy task, most will choose the easier task. If there is more than one path, most take the “path of least resistance” (resistors in parallel).

Resistors in parallel PHY 201 (Blum)50

PHY 202 (Blum)51 Parallel Two resistors are in parallel if the top ends of the two resistors are connected by wire and only wire and likewise for the bottom ends. A charge will pass through one or the other but not both resistors.

PHY 202 (Blum)52 Resistors in parallel The voltage across the resistors is the same V 1 = V 2 = V ab (you use up the same energy either way) The current is split between the resistors I = I 1 + I 2 R1R1 R2R2

PHY 202 (Blum)53 Equivalent resistance (parallel) I = I 1 + I 2 V ab = V1V1 + V2V2 R eq R1R1 R2R2 1 = R1R1 R2R2 V’s are same, so they cancel

PHY 202 (Blum)54 Resistors in parallel Resistors in parallel add reciprocally. The equivalent resistance will be smaller than either individual resistance. It is always easier if one has a choice of what one has to go through.

PHY 202 (Blum)55 Equivalent Resistance

PHY 202 (Blum)56 1 = R eq R1R1 R2R2 If R 1 = 2.3 and R 2 = 3.4, then do the following on the calculator: 2.3 → 1/x → + → 3.4 → 1/x → = → 1/x → 1.37

PHY 202 (Blum)57 Fire in a theater analogy If it bothers you that the resistance of two resistors in parallel is lower than either resistor, consider the following. A fire starts in a packed theatre and there is one door through which everyone must exit. It’s a difficult task to get everyone out. A second exit is found, the second exit is narrower and fewer people can use it. However, the theater can be emptied much faster using two exits than one – even if a given person can only use one of the exits.

PHY 202 (Blum)58 Series/Parallel Recap Series Resistors in series have the same current. Their voltages add up to the total voltage. R s = R 1 + R 2 Parallel Resistors in parallel have the same voltage. Their currents add up to the total current. 1/R p = 1/R 1 +1/R 2

PHY 202 (Blum)59 Serial and parallel connections A connection is said to be serial if all of the bits entering follow exactly the same path, bits then arrive one- by-one. A connection is said to be parallel if there are a set of paths, bits can then take different paths and groups of bits can arrive simultaneously.

PHY 202 (Blum)60 Multi-meter A multi-meter can serve as a voltmeter, ammeter or ohmmeter depending on its setting. To measure the voltage across a resistor, the voltmeter is placed in parallel with the resistor. To measure the current through a resistor, the ammeter is placed in series with the resistor. To measure the resistance of a resistor, the resistor is removed from the circuit and each end is connected to an end of the ohmmeter.

PHY 202 (Blum)61 Voltmeter in parallel with 1- k Resistor

PHY 202 (Blum)62 Ammeter in series with 1- k Resistor

PHY 202 (Blum)63 Ohmmeter measuring resistance of 1- k and 2 -k resistors in series

PHY 202 (Blum)64 Checking continuity A wire or cable is metal (a conductor) on the inside and thus has a low resistance. A broken cable has a high resistance. To check a cable, remove the cable, set the multi-meter to ohmmeter Check each wire for “continuity” (should find a low resistance).

PHY 202 (Blum)65 Heat A basic principle of physics is that energy is conserved, that is, energy is never lost or gained but only rearranged and put in different forms. When we have a simple resistor circuit, the potential energy that was in the battery becomes heat which is another form of energy.

References Chapters 3 through 5 of Digital Principles (Tokheim) m rads/courses/phys202fall96/?D=A PHY 201 (Blum)66