nsmission of data, the marking condition is used to denote the binary state ONE and the spacing condition is used to denote the binary state ZERO. Besides EIA-232 there are many other interface standards issued by EIA, CCITT, U.S. federal standards, U.S. military standards, and ISO. Each defines the DTE-DCE interface. Several of the more current standards are briefly described in the following. EIA-530 (Ref. 7) is a comparatively recent standard developed by the EIA. It provides for all data rates below 2.1 Mbps and it is intended for all applications requiring a balanced electrical interface. 7 It can also be used for unbalanced operation. Let us digress for a moment. An unbalanced electrical interface is where one of the signal leads is grounded; for a balanced electrical interface, no ground is used. EIA-530 applies for both synchronous and nonsynchronous (i.e., start-stop) operation. It uses a standard 25-pin connector; alternatively it can use a 26-pin connector. A list of interchange circuits showing circuit mnemonic, circuit name, circuit direction (meaning toward DCE or toward DTE), and circuit type is presented in Table 10.2. EIA-530 is more properly called ANSI/EIA/TIA-530-A. 10.9 DIGITAL TRANSMISSION ON AN ANALOG CHANNEL Table 10.2 EIA-530 Interchange Circuits Circuit Mnemonic CCITT Number Circuit Name Circuit Direction Circuit Type Signal common Common Signal common Transmitted data To DCE Received data From DCE Request to send To DCE Clear to send From DCE Received line signal detector From DCE Ready for receiving To DCE Ring indicator From DCE Control DTE ready From DCE 108/1,/2 DTE ready To DCE Transmit signal element timing To DCE (DTE source) Transmit signal element timing From DCE Timing (DCE source) Receiver signal element timing From DCE (DCE source) Local loopback To DCE Remote loopback To DCE Test mode From DCE Source: Ref.7. 10.9 DIGITAL TRANSMISSION ON AN ANALOG CHANNEL 10.9.1 Introduction Two fundamental approaches to the practical problem of data transmission are (1) to design and constr