Data Acquisition

interface requirements when connecting data terminals to the Bell telephone system. Almostimmediately, shortcomings resulted in minor revisions to the standard, becoming EIA-232-Cstill widely used today. The current revision is EIA/TIA-232-E (1991), which brings it intoline with the international standards CCITT V.24, CCITT V.28, and ISO-2110. Some userscontest the statement that EIA-232 is a ‘standard’, because its interpretation has beenresponsible for many problems in interfacing equipment from different manufacturers. Itshould be emphasized that EIA-232 standard defines the electrical and mechanical details ofthe interface and does not define a protocol.The EIA-232 standard consists of three major components, which define:• Electrical signal characteristicsElectrical signals such as the voltage levels and grounding characteristics of theinterchange signals and associated circuitry.• Interface mechanical characteristicsThe mechanical characteristics of the interface between the DTE and DCE. Thissection dictates that the interface must consist of a plug and socket, and that thesocket will normally be on the DCE. The familiar DB-25 connector is specifiedtogether with a smaller 26-pin alternative connector.• Functional description of the interchange circuitsThis section defines the function of the data, timing and control signals used at theinterface between DTE and DCE. Very few of the definitions in this section arerelevant to applications for data communications for data acquisition and control.6.2.1 Electrical signal characteristicsThe RS-232 standard is designed for the connection of two devices: data terminal equipment(DTE), such as a computer or printer, and data communications equipment (DCE), such as amodem. DCEs are now also called data circuit-terminating equipment in the RS-232-D/Estandard. This definition was required since the RS-232 standard is often used to interfaceitems of equipment that are not communication devices.A typical connection between a computer (DTE), which transmits data on pin 2 andreceives data on pin 3 of a 25-way DB connector, to a stand-alone controller (DCE) thatreceives data on pin 2 and transmits data on pin 3 is shown in Figure 6.5.Figure 6.5Connections between a computer (DTE) and a stand-alone controller (DCE)