| 摘要 |
The points have axle passage detectors (2, 3, 4) at both entry and exit which initiate pulse circuits (5, 6) when interrogated by a clock pulse (T1). The pulse initiated by the exit detector is applied via an OR gate (9) to one input of a switch circuit (13) and via a delay circuit (12) to the other ('enable') input. The output from this is in turn fed to a similar switch whose other input receives impulses via another OR gate from either the entry detector or a second clock pulse which is applied together with the output from the second switch to an AND gate. An axle-counting circuit (11) receives pulses from this gate, from the exit detector, from the bistable switch and from the count comparator (17). If the exit count exceeds the entry count, absence of an 'enable' signal to the second switch prevents forwarding to the axle counting circuit and consequent blocking. |
