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by Robin L. Minnear You could go out and buy a logic comparator for $400 to $600 . And even at that price you may find it extremely useful. This month and next month I will describe how to build a functional logic comparator. This month will be a discussion of the actual logic comparing circuitry and the conclusion next month will be about the pulse stretching and indicator circuitry. A Logic Comparator is a device that will allow you to determine if a chip is bad on your logic board by comparing it to a known good chip of the same type. All you have to do is plug a good chip (the same type as you wish to check) into the socket provided on the comparator. Next you attach the test clip from the comparator to the chip on your logic board that you wish to test. Attach power leads from the comparator to the logic board. Power up the logic and the comparator will tell you if the chip is good or bad . The actual comparing portion of the logic comparator uses a TTL 7486 chip. A 7486 is a Quad I /'I 2 /3 3 IZ lf II 5 10 6 7 'I g Gu~o £xCLus fllt:. f1c,uru: l Oil purpose is to program the comparator function. Our logic devices consist of inputs and outputs. If pin 8 in Figure 3 is an output pin, you want the switch to be in position "B." This will allow the outputs to be compared by the Exclusive Or. If Exclusive Or device. This means that there are four individual exclusive or devices in one package as per Figure 1. Each exclusive or gate is made up of two inputs and one output . According to the truth-table in SYM~OL TR UTt-\ 11\ 8 LE. C\Ji~\JT fr §. .f: AB~~ 1 y ~ ~~ t~ I~ LO HI ZF //l/PUT..5 UCLUSIV£. OR. .4 HI /fl ftrJ./:j HI Lo B ft/2.£ THE. .SAl'Y1E- 1 o 11-r-PVT '( I 5 l-0. r F -4 A/II.£ 8 ~RE DJFFEQE.NT1 Y /S H-1. Figure 2: if the information on its inputs are the same, both HI or both LO, the output is LO. If the information on its inputs are different, one is HI and one is LO, the output is HI. By taking the input information of the chip you are testing and applying it to the same inputs of the known good chip, you can determine if the chip under test is good or bad. The logic comparator design presented in these articles is good for checking TTL and DTL devices up to sixteen pins. This can be easily expanded if you desire. The basic comparator circuit is illustrated in Figure 3. As you can see, the logic comparator consists of six basic parts. 1. This is a socket on the comparator where you place a known good chip that is the same type as the chip you want to test. 2. This is an IC clip that connects to the chip under test on your logic board . The clip makes contact with all sixteen legs of the chip and is connected to the comparator via cable. 3. This is a SPOT switch. Its pin 8 is an input (or power) pin, then you would put the switch in position "A." This would put the same signal conditions that are on the device to be tested, on the corresponding input of the known good chip. This places the known good chip under the same circumstances as the one on your logic board. You would also disregard the comparator reading of this pin. Your known good chip derives its power from the logic board. 4. This is the Exclusive Or gate that compares the two chips outputs. Its output is LO when its inputs are the same or the compare is "Good." Its output is HI when its inputs are different or a "Bad" compare. 5&6 . Next month is a complete discussion of sections five and six. Section five is a circuit that stretches the pulse put out by the Exclusive Or. Section six is the LED display. CIRCUIT DESCRIPTION ;i The comparator described in ~ this article is good for checking TTL ;c and DTL chips up to sixteen pins. l'l'I For checking chips with more than ; 39