| 摘要 |
PCT No. PCT/EP88/00533 Sec. 371 Date Sep. 13, 1990 Sec. 102(e) Date Sep. 13, 1990 PCT Filed Jun. 16, 1988 PCT Pub. No. WO89/12928 PCT Pub. Date Dec. 28, 1989.The switch condition of two switches (S1, S2) connected in parallel to a single transmission line (11) is detected by the signals output from two Schmitt triggers (10a, 10b) which are arranged to operate at different switching threshold levels by virtue of the output (WA) of one of the triggers (10a) being connected to the threshold level control input (F) of the other trigger (10b). Closure of one of the switches (S1 or S2) causes a reference voltage (UR) to the applied to the inputs of the triggers (10a, 10b) which is turn causes the upper threshold trigger (10a) to operate. Closure of both switches (S1 and S2) reduces the level of the voltage applied to the inputs of the triggers which is turn causes the lower threshold trigger to operate. Preferably, the signal input to the lower threshold trigger is converted into a current prior to application to the trigger due to the relatively small difference in threshold levels.
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