Practical SCADA for Industry David Bailey - FER-a

x Contents
6 Modems 176
6.1 Introduction 176
6.2 Review of the modem 176
6.2.1 Synchronous or asynchronous 178
6.2.2 Modes of operation 179
6.2.3 Components of a modem 180
6.2.4 Modem receiver 180
6.2.5 Modem transmitter 181
6.3 The RS-232/RS-422/RS-485 interface standards 182
6.3.1 The RS-232-C interface standard for serial data communication 182
6.3.2 Electrical signal characteristics 183
6.3.3 Interface mechanical characteristics 185
6.3.4 Functional description of the interchange circuits 185
6.3.5 The sequence of asynchronous operation of the RS-232 interface 186
6.3.6 Synchronous communications 187
6.3.7 Disadvantages of the RS-232 standard 188
6.3.8 The RS-422 interface standard for serial data communications 188
6.3.9 The RS-485 interface standard for serial data communications 190
6.4 Flow control 191
6.5 Modulation techniques 191
6.5.1 Amplitude modulation (or amplitude shift keying) 192
6.5.2 Frequency modulation (or frequency shift keying — FSK) 192
6.5.3 Phase modulation (or phase shift keying (PSK)) 192
6.5.4 Quadrature amplitude modulation (or QAM) 193
6.5.5 Trellis coding 194
6.5.6 DFM (direct frequency modulation) 195
6.6 Error detection/correction and data compression 196
6.6.1 MNP protocol classes 196
6.6.2 Link access protocol modem (LAP-M) 197
6.6.3 Data compression techniques 198
6.7 Data rate versus baud rate 201
6.8 Modem standards 202
6.9 Radio modems 203
6.10 Troubleshooting the system 207
6.10.1 Troubleshooting the serial link 207
6.10.2 The breakout box 208
6.10.3 Protocol analyzer 208
6.10.4 Troubleshooting the modem 209
6.11 Selection considerations 210
7 Central site computer facilities 212
7.1 Introduction 212
7.2 Recommended installation practice 212
7.2.1 Environmental considerations 212