Hardware PropertiesInterfacing devices
Types of UARTs
Loop Back Plug
Almost all digital devices we use require TTL or CMOS logic levels. That's why the first step when trying to connect the device to RS-232 port is transformation of RS-232 levels ack into 0 and 5 Volts. As we have already said, this is done by RS-232 Level Converters.
Two common RS-232 Level Converters are the 1488 RS-232 Driver and the 1489 RS-232 Receiver. Each package contains 4 inverters of the one type, either Drivers or Receivers. The driver requires two supply rails, +7.5 to +15v and -7.5 to -15v. This may pose a problem in many instances where only a single supply of +5V is present. However the advantage of these IC's is their cheapness.
Another device is the MAX-232. It includes a Charge Pump, which generates +10V and -10V from a single 5v supply. This I.C. also includes two receivers and two transmitters in the same package. This is useful in many cases when you only want to use the Transmit and Receive data Lines. You don't need to use two chips, one for the receive line and one for the transmission. However this convenience is expensive, but compared with the price of designing a new power supply it is very cheap.
There are also many variations of these devices. The large value of capacitors are not only bulky, but also expensive. That's why other devices are available which use smaller capacitors and even some with built-in capacitors. (Note : Some MAX-232's can use 1 micro farad Capacitors). However the MAX-232 is the most common, and thus this RS-232 Level Converter will be used in our examples.
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