Serial Communication using C# and Whidbey 

c# 2.0 serial port communication串口编程实例
 使用SerialPort类设计串口通讯程序

About Serial Port (Hardware)

The serial port on your PC is a full-duplex device meaning that it can send and receive data at the same time. In order to be able to do this, it uses separate lines for transmitting and receiving data. Some types of serial devices support only one-way communication, and therefore, use only two wires in the cable - the transmit line and the signal ground.

Data Transfer

In serial communication, a byte of data is transferred through a single wire one bit at a time. The packets contain a start bit, data, and stop bit. Once the start bit has been sent, the transmitter sends the actual data bits. There may either be 5, 6, 7, or 8 data bits, depending on the number you have selected. Both receiver and the transmitter must agree on the number of data bits, as well as the baud rate.

Null Modem

A Null Modem cable simply crosses the receive and transmit lines so that transmit on one end is connected to receive on the other end and vice versa. In addition to transmit and receive, DTR & DSR, as well as RTS & CTS are also crossed in a Null Modem connection.

Here is the pin diagram:

As we are not using any handshake, we will use three wire connections in which we will connect pin 2 of one connector to pin 3 of the other. For both the connectors, connect pin 5 (ground) of both the connectors with each other (common ground).

If you want, you can use only one PC as both transmitter and receiver for this communication. Then, just take a DB 9 connector and a small wire, and connect pin no 2 and 3 using the wire that will connect a loop back. That is, whatever you will send, the same data you will be receiving.