在程控交换机中,电话机“R”键具有转移呼叫、三方通话、恶意追查等功能。实际上它是一个时间参数为T_0的负脉冲产生电路,其波形如图1。“R”键电路一般由“R”键开关、电容C_T和电阻R组成,这里以HA868(Ⅴ Ⅲ)P/TSD话机为例加以说明,其相关电路见图2。电路中负脉冲的时间参数由馈电电压、电容C_T(这里为C_X)、电阻R(这里为R_(109))决定。在目前较为普遍使用 In program-controlled switches, the phone “R” key with transfer calls, three-way calling, malicious tracing and other functions. In fact it is a time pulse T_0 negative pulse generator circuit, the waveform shown in Figure 1. The “R” key circuit is generally composed of “R” key switch, capacitor C_T and resistor R, here HA868 (Ⅴ Ⅲ) P / TSD phone as an example to illustrate the circuit shown in Figure 2. Negative pulse circuit time parameters by the feed voltage, capacitance C_T (here C_X), resistance R (here R_ (109)) decision. In the current more common use