Data Acquisition

At the RS-232 receiver the following signal voltage levels are defined:• +3 V to +25 V for transmission of logic 0• –3 V to –25 V for transmission of logic 1• +3 V to –3 V for an undefined logic levelTo meet these voltage requirements at the receiver and to overcome any voltage drops thatoccur along the communications lines, the RS-232 transmitter must produce slightly highervoltages.These are in the range:• +5 V to +25 V for transmission of logic 0• –5 V to –25 V for transmission of logic 1• +5 V to –5 V for an undefined logic levelIn practice, many EIA-232 transmitters operate very close to their margin of safety, e.g. at+7 and –7 volts. This can be acceptable for short cable runs, where it is hoped that there willbe no voltage problems. Unfortunately, increased error rates can be expected at the receiverbecause of induced external interference voltages.The voltage levels associated with the internal electronics of DTE and DCE devices arecommonly –5 V to +5 V and are therefore not directly compatible with the signal levelsassociated with the communications interface. Consequently, at the transmitting end, a LineDriver is necessary in each data and control line to amplify this voltage to the higher levelrequired on the EIA-232 interface. Modem power supplies, such as those used in PCs, usuallyhave a standard +12 V to –12 V voltage output that can then be used for the line driveroutput. This falls within the voltage range specified by the EIA-232 standard and is thevoltage level most commonly used these days.At the receiving end, a Line Receiver is necessary for each data and control line to reducethe voltage level to the –5 V to +5 V level required by the internal electronics.Figure 6.6EIA-232 transmitters and receiversThe EIA-232 standard defines twenty-five (25) electrical connections, which are eachdescribed in more detail in Section 6.2.3.