Decibel table , Connectors and connections and Modem connector pin numbers

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Decibel table

The decibel figures are in the centre column: figures to the left represent decibel loss, and those to the right decibel gain. The voltage and current figures are given on the assumption that there is no change in impedance.

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Connectors and connections

Data interchange by modems

When transmitting and receiving data across telephone or other circuits, the equipment which actually generates and uses the data (e.g. a computer or VDU terminal) is known as data terminating equipment (DTE). The equipment which terminates the telephone line and converts the basic data signals into signals which can be transmitted is known as data circuit-terminating equipment (DCE). As far as the user is concerned the interface between DTE and DCE is the most important. CCITT recommendation V24 defines the signal interchanges and functions between DTE and DCE; these are commonly known as the 100 series interchange circuits:

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Modem connector pin numbers

The connectors used with 100 series interchange circuits and its pin assignments are defined by international standard ISO 2110 and are (for modems following the CCITT recommendations V21, V23, V26, V26bis, V27 and V27bis) as follows:

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Automatic calling

A similar series of interchange circuits is defined in CCITT recommendation V25 for automatic calling and answering between modems over the telephone network. This is the 200 series of interchange circuits.

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EIA 232

The EIA equivalent of CCITT V24 interface is the EIA 232 specification, which similarly defines the electrical interface between DTE and DCE. Although the two have different designations, they are to all practical purposes equivalent. The EIA 232 interchange circuits are:

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EIA 449

The EIA 232 standard, although the most common, is by no means perfect. One of its main limitations is the maximum data rate — 18.2 Kbaud. Various improved interchange circuits (EIA 422, EIA 423) have been developed. The EIA 449 standard is capable of very fast data rates (up to 2 Mbaud):

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Centronics interface

Most personal computers use the Centronics parallel data transfer to a printer. The pin connections of the connector, abbreviations and signal descriptions are shown.
All signals are standard TTL, although not all signals necessarily exist in any given interface.

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EIA 232/EIA 449/V24 comparison

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