问题 问答题 简答题

比较 RS-232 与RS-449 的电气特性。

答案

参考答案:

RS-232规定逻辑“1”的电平为:-15~-5,逻辑“0”的电平为+5~+15。两设备的最大距离也仅为15米,而且由于电平较高,通信速率反而影响。接口通信速率小于等于20Kbps。RS-422由于采用完全独立的双线平衡传输,抗串扰能力大大增强。又由于信号电平定义为正负6伏,当传输距离为10m时,速率可达10Mbps;当传输距离为1000m时,速率可达100Kbps。

RS-423,电气标准是非平衡标准。它采用单端发送器和差动接收器。当传输距离为10m时,速率可达100Kbps;当传输距离为1000m时,速率可达10Kbps。

完形填空

  Everyone can feel stressed out because of too many things happening in your life. Teenagers, however, have __36__ opportunities to get stressed than people in any other age group. Being a teenager is hard.

You are not a children any more, but you are not __37__, even though you have to deal with some grow-up problems and made decisions by __38__. Families can be one of the biggest causes(原因)of stress, such as problems of arguing with __39__ or brothers or sisters. Teenagers also have lots of stress from __40__, either from their teachers or from their classmates. Some teenagers also feel __41__ about choosing their education after high school. Getting a place at university can be very difficult and some can’t __42__ to go to university. It is hard for some teenagers to get a job after they graduate. There are so many young people finishing school __43__ not enough jobs for them. Though there is a lot of stress, you can find many ways to __44__ it. Talking to people is one of the best ways to deal with stress. It may sound __45__, but it is helpful.

小题1: A. more                        B. fewer                   C. many

小题2:A. a baby                      B. a teenager               C. an adult

小题3:A. yourself                   B. himself                    C. myself

小题4:A. teachers                   B. parents                    C. classmates

小题5:A. family                      B. society                   C. school

小题6: A. relaxed                     B. stressed                   C. excited

小题7: A. afford                      B. offer                      C. refuse

小题8:A. and                        B. but                      C. or

小题9: A. agree with                  B. talk with                  C. deal with

小题10: A. simple                      B. difficult                   C. good

单项选择题