Types of Serial Cablesby Saifuddin Abdullah
Data communication between digital devices is often done through serial cables, which are used to transfer data between devices that use bit-by-bit or serial port communication techniques. Most types of serial cables use RS-232 (Recommended Standard-232) standard interface for communications, which mainly specifies that data must be sent in a predefined sequence of bits. Generally, serial cables are categorized according to their interfaces and standards.
This common serial cable is used between a DTE and DCE--that is, data terminal equipment and data communications equipment respectively. In communications terminology, DTE refers to terminating/initiating point of session and DCE means forwarding point of a communication session. These cables offer both 9-25 and 25-25 pin connections at transmit-receive end connectors. DTE-DCE cables are normally used to establish a connection between computers and modem cards. In this scenario, a computer acts as a DTE and modem card functions as DCE.
Null-modem is the ideal standard cable to connect two PCs using null-modem configuration. It is called null-modem because it involves no DCE (modem) between DTE-DTE sessions. Moreover, this process uses a certain signal quantity to ensure connection stability between two devices, which is known as "handshaking." A null-modem medium is configured by a serial cable connected to a null-modem adapter, having transmit and receive lines cross-linked. Most common null-modem cables contain 9-9, 9-25 and 25-25 pin configuration at their connectors.
This cable is nearly identical to basic DTE-DCE cable, except that no handshaking process is involved between computer and modem. This cable is also available in 9-25 and 25-25 pin configurations, similar to basic DTE-DCE. A limitation of a non-handshaking procedure is the absence of acknowledgment from the DTE end.
The V.35 crossover interface is used to connect DTE-DTE and DCE-DCE nodes. It also provides enhanced data rate as high as 64 kbps, by providing 34-34 pin configurations at both transmitter and receiver ends. This particular medium is a mixture of RS-232 and RS-449 standards. Its successors in RS-232 and RS-449 standard domains include V.28 and V.11, respectively.
Serial printer cables are used to establish connection between a computer and printer through a serial port. This cable is configuration specific; that is, a 9-25 pin cable is used when both printer and computer act as DTE and a 25-25 pin cable is used when computer and printer act as DTE and DCE, respectively. When both devices are DTE, a null-modem cable can work; however, if a serial printer is connected to a computer through it, it is ineffective.