TRUTH TABLES AND EXERCISESDOE-HDBK-1016/2-93Logic DiagramsPR-05Page 18Rev. 0Answersto example 1.1.a. 2b. 3c. 1d. 42.The safety signal must be received for greater than 10 minutes before it will passthrough the time delay. If the safety signal is removed before 10 minutes has elapsedno signal will be passed to Gate 2.3.Gate 2 will turn on when the hand-switch is in the AUTO position and a safety signalhas been received for greater than 10 minutes.4.If flow switch (FS) 30-38 senses less than 20,000 cfm, 45 seconds after the fan hasstarted, or the same condition exists on the 1B-B fan, the alarm will sound.5.AND Gate 2 prevents the on signal from passing until a safety signal is also received(>10 minutes).6.Ten minutes after receiving the safety signal, the fan started. At 12 minutes, removingthe safety signal only removes the continuous start signal to the fan. The fan willcontinue to run until the hand switch is placed in the stop position. Further, with theremoval of the safety signal, the fan will remain stopped when the hand switch springreturns to the AUTO position. Note that if the hand switch is placed in the stopposition while the safety signal is present, the fan will stop, but will restart as soon asthe switch spring returns to the AUTO position.7.It can be started by two signals - START and AUTO plus a safety signal.It can be stopped by one signal - STOP (but will only remain stopped if no safety signalis present or the switch is held in the stopped position).Answersto example number 2.1.a. 1 & 4b. 2c. 32.Right input is - on - this is because the hand control switch is in the AUTO position, andthe AUTO switch contacts are made up, resulting in an on signal. Therefore the hand-switch CLOSE position contacts are open, resulting in an off signal. The off signal isreversed in the NOT gate and becomes an on signal.
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