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first scan of the PLC -indicating the PLC has just been turned on time since an input turned on/off -a delay count of events -to wait until set number of events have occurred latch on or unlatch -to lock something on or turn it off The common theme for all of these events is that they are based upon one of two questions "How many?" or "How long?". An example of an event based device is shown in Figure 9.1. The input to the device is a push button. When the push button is pushed the input to the device turns on. If the push button is then released and the device turns off, it is a logical device. If when the push button is release the device stays on, is will be one type of event based device. To reiterate, the device is event based if it can respond to one or more things that have happened before. If the device responds only one way to the immediate set of inputs, it is logical. plc timers -9.2 e.g. A Start Push Button Push Button Push Button Device A latch is like a sticky switch -when pushed it will turn on, but stick in place, it must be pulled to release it and turn it off. A latch in ladder logic uses one instruction to latch, and a second instruction to unlatch, as shown in Figure 9.2. Easy Access To All Pages 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 |
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