Equipment Used in Communications

STANDARDIZED INTERFACES

Most of the devices used for data processing have limited transmission capacity. Generally, they generate a digital signal, so it is rare that these devices (terminals and computers) connect directly to the transmission network. End devices are generally called data terminal equipment (DTE). A DTE makes use of the transmission medium through the use of data circuit-terminating equipment (DCE).

On the one hand, the DCE is responsible for transmitting and receiving bits, one at a time, through the transmission medium or network. On the other hand, the DCE must interact with the DTE.

In general, this requires that both data and control information be exchanged. This is done through a set of wires called exchange circuits. For this scheme to work, a high degree of cooperation is needed as the two DCEs exchanging signals over the transmission line or network must understand each other. That is, the receiver of each DCE must use the same coding scheme and the same transmission rate as the transmitter at the other end. Furthermore, each DTE-DCE pair must be designed to work cooperatively. To make things easier in data processing, important features have been developed that are:

• Mechanics
• Electrical
• Functional
• Procedure

MECHANICS: It deals with the physical connection between the DTE and DCE. The control and data exchange circuits are embedded in a cable with a connector, male or female at each end. The DTE and DCE must have different gender connectors at each end of the cable, this configuration is analogous to power supply cables.

ELECTRICAL: They are related to voltage levels and timing. Both the DTE and the DCE must use the same code, must use the same voltage levels, and must use the same duration for the signal elements. These characteristics determine the transmission speed as well as the maximum distances that can be achieved.

FUNCTIONAL: They specify the functions that are performed through each of the exchange circuits. The functions to be performed can be classified into four main categories: Data, Control, Timing and Ground.

PROCESS: Specifies the sequence of events that must occur in data transmission, based on the functional characteristics of the Interface.

PARALLEL PORT

There are two basic methods for data transmission in modern computers. In a serial data transmission scheme, one device sends data to another one bit at a time over a wire. On the other hand, in a parallel data transmission scheme, one device sends data to another at a rate of n number of bits through n number of wires at a time. It would be easy to think that a parallel system is n times faster than a serial system, however this is not true, basically the main impediment is the type of cable used to interconnect the equipment. While a parallel communication system can use any number of wires to transmit data, most parallel systems use eight data lines to transmit one byte at a time. As with everything, there are exceptions, for example the SCSI standard allows data transfer in schemes ranging from eight bits to thirty-two bits in parallel. In this article we will concentrate on eight-bit transfers since this is the parallel port configuration of a PC.

A typical parallel communication system can be one-way (unidirectional) or two-way (bidirectional). The simplest mechanism used in a PC parallel port is the unidirectional type and it is the one that we will analyze first. We distinguish two elements: the transmitting part and the receiving part. The transmitting party places the information on the data lines and informs the receiving party that the information (data) is available; then the receiving party reads the information on the data lines and informs the transmitting party that it has taken the information (the data). Note that both parties synchronize their respective access to the data lines, the receiving party will not read the data lines until told to do so by the transmitting party while the transmitting party will not put new information on the data lines until the transmitting party does. receiving party removes the information and indicates to the transmitting party that it has already taken the data, this coordination of operations is called an agreement or understanding. Well, in these technological fields it is advisable to use certain words in English that ironically allow us a better understanding of the concepts treated. I repeat: the coordination of operations between the transmitting party and the receiving party is called handshaking, which in Spanish is the act with which two parties agree, that is, they shake hands.

handshaking

To implement handshaking, two additional lines are required. The stroboscope line (in English strobe) is the one used by the transmitting party to indicate to the receiving party the availability of information. The acknowledgment line is what the receiving party uses to tell the transmitting party that it has taken the information (the data) and is ready to receive more data. The parallel port provides a third handshaking line called busy in English, this can be used by the receiving party to indicate to the transmitting party that it is busy and therefore the transmitting party should not try to put new information on the lines. of data. A typical data transmission session looks like the following:

Transmitting part:

• The transmitting party checks the busy line to see if the receiving party is busy. If the busy line is up, the transmitting party waits in a loop until the busy line is down.
• The transmitting part places the information on the data lines.
• The transmitting part activates the strobe line.
• The transmitting party waits in a loop until the acknowledge line is active.
• The transmitting part inactivates the strobe line.
• The transmitting party waits in a loop until the acknowledge line is idle.
• The transmitting party repeats the above steps for each byte to be transmitted.

Receiving party:

• The receiving party inactivates the busy line (assuming it is ready to receive information).
• The receiving party waits in a loop until the strobe line is active.
• The receiving party reads the information from the data lines (and if necessary, processes the data).
• The receiving party activates the acknowledge line.
• The receiving party waits in a loop until the strobe line is inactive.
• The receiving party inactivates the acknowledge line.
• The receiving party repeats the above steps for each byte it is to receive.

One must be very careful in following these steps, both the transmitting and receiving parties coordinate their actions in such a way that the transmitting party will not try to put multiple bytes on the data lines, while the receiving party must not read more data than those sent to it by the transmitting party, one byte at a time.