week 5

Week 5

When you press the buttons on the keypad, a connection is made that generates two tones at the same time. A "Row" tone and a "Column" tone. These two tones identify the key you pressed to any equipment you are controlling. If the keypad is on your phone, the telephone company's "Central Office" equipment knows what numbers you are dialing by these tones, and will switch your call accordingly. If you are using a DTMF keypad to remotely control equipment, the tones can identify what unit you want to control, as well as which unique function you want it to perform. While the engineers were designing this protocol, they decided to throw in a few more "special purpose" tone groups. You don't normally see these extra four buttons on telephones, but they are alive and being employed for communications signaling. For lack of imagination, the engineers called the four extra digits "A B C D". These all use the same row frequencies as a standard keypad, but they have an additional column tone. The extra codes are very useful in preventing standard telephone codes from being used to control remote devices, and can give you override status when used correctly in a two-way radio system. To send the digits quickly (40/40) you need to be using an automated encoder since you could not press each button by hand that fast. An automated encoder is also called a "Store and Forward" encoder. You "Store" the Dtmf code into the device when you program it, then "Forward" it to send the entire Dtmf code as one complete Dtmf string of numbers and characters.