- Serial signals usually use 9 pin connectors.
Synchronous and Asynchronous transmission both use NRZ, or Non Return to Zero, encoding to represent the bits. This uses two voltage levels. A bit value of one is represented by a high voltage, and a bit value of zero is represented by a low voltage. The highest transition density is sequence of ones and zeros, or an upward transition for one bit followed by a downward transition for the next bit. Each cycle, or hertz, contains two bits. Non Return to Zero coding is characterized by a bit density of two bits per hertz. The minimum bit density is a long string of similar bits and there would be no transitions. The transmit clock and receive clock might wander away from each other. Non Return to Zero encoding does not provide clock recovery.
- Synchronous signals must be coordinated.
Synchronous signals transmit bits at regular intervals all the time. One special bit pattern, the flag, separates the information into frames. If there is no information to transmit, flags are transmitted continuously. The bits must be synchronized from the transmitter to the receiver. Since NRZ does not provide clock recovery, a separate line must be provided to give clock signaling. Synchronous communication is normally used on serial lines between routers. Synchronous communications typically run at speeds of between 64 Kbps and 2 Mbps.
- Asynchronous signals transmit whenever ready.
When there is nothing to send on an asynchronous line, the line voltage is held at the low level. When one end wants to transmit some information, it will begin the transmission with one or two start bits. This allows the receiver to synchronize the clock in order to correctly interpret the remaining bits. The data bits are then sent, usually eight bits, followed by a stop bit. Each eight bits represents one character. There is no need to synchronize transmission and so the term asynchronous applies. Personal computers use asynchronous communications by default. Asynchronous lines run much slower. Speeds from 9.6 Kbps to 115.2 Kbps are common.