Null Modem

Null Modem configurations are used to connect two DTE with a wiring diagram that can be seen in the image below. In this case it only takes three wires between the DTE, which is to TxD, RxD and Gnd. The way it works is how to make the computer into thinking that the computer communicates with the modem (DCE) rather than with other computers.



In the picture above shows that the foot DTR (Data Terminal Ready) is connected to the DSR (Data Set Ready) and also to the CD (Carrier Detect) on each computer, so that when the DTR signal is activated then DSR and CD signals also active (Modem concepts Virtual Modem or pseudo). Because the computer in this case sending data at the same speed, the flow control (flow control) is not required so that the RTS (Request To Send) and CTS (Clear to Send) on each computer connected to each other.

Data transmission in RS232
The RS-232 communication with PC is asynchronous communication. Where clocknya signal is not sent along with the data. Each data synchronized using the internal clock on each side. Figure 2.6 Format of transmission of the RS232 Data byte transmitted in the above format is 8 bits, before the data is transmitted it will be preceded by a start bit to logic 0 (0 volts), then 8 bits of data and is terminated by a stop bit to logic 1 ( 5 Volt).

Advantages of Using Serial Communications
Serial communications interface offers several advantages compared with parallel communication, including:

• Cables for serial communication can be longer than parallel.
The data in the serial communication is sent to logic '1 'as the voltage of -3 s / d and -25 volts for logic '0' as the voltage of +3 s / d +25 volts, thus the voltage in serial communication has a maximum voltage swing of 50 volts, while the parallel communication is only 5 volts. This causes disruption in the wiring length is more easily overcome than the parallel.

• The number of serial cable less.
Two sets of a remote computer with only three cables to null modem configuration, ie, TxD (send channel), RxD (receive channel) and the Ground, but if you use parallel communication there will be twenty to twenty-five wires.

• Serial communications can use the free air as the transmission medium.
In the serial communication is transmitted one bit at a time so that when the transmission using a media-free air (free space) then the receiver section will not appear difficult to reconstruct the bits of bits transmitted.

• Serial communications can be applied to communicate with the microcontroller.
It only takes two main pin TxD and RxD (outside the reference ground).