Auto dialers are alarms that send signals when equipment malfunctions, a process variable exceeds a predefined limit, or another alarm-triggering event occurs. Both hard-wired and wireless products are available. Hard-wired auto dialers require electrical wires or telephone cables. Wireless auto dialers use electromagnetic waves instead. With both types of auto dialing devices, the alarm signal is delivered to a telephone, cell phone, fax machine, pager, and/or computer. Some auto dialers are used in the water and wastewater industry. Others are designed for chemical and petrochemical facilities, factories and warehouses, bridges, power plants, and other secure facilities. Hard-wired or landline auto dialers are designed to be connected to the public switched telephone network (PSTN). When an equipment failure or process deviation occurs, these devices place telephone calls to plant personnel. Some auto dialers describe problems and conditions by using prerecorded voice messages. Personnel can then acknowledge these alarms and press specific keys on their telephone keypads to adjust plant equipment. In this way, remote operators in water treatment and petrochemical processing facilities can adjust pumps and valves remotely. Many auto dialers accept digital inputs and interface to personal computers (PCs) and programmable logic controllers (PLCs). By using industrial serial networking protocols such as Modbus® (Schneider Automation), auto dialers can report exceptions in PLC control registers. Typically, Modbus is used to connect a supervisory computer with a remote terminal unit (RTU) in a larger supervisory control and data acquisition system (SCADA). In this way, auto dialers can support SCADA systems in a variety of process control applications. Typically, autodialers for larger SCADA systems support Modbus TCP/IP and are designed for use open protocols such as Ethernet. Cellular auto dialers use wireless technologies instead of telephone lines, making these devices well-suited for remote locations without hard-wired networks. Wireless auto dialers do not require dedicated phone lines either. Often, wireless auto dialers are installed at bridges, oil wells, and lift stations. By incorporating cell phone technology, these devices support voice alerts as well as SMS text messaging. Some wireless or cellular auto dialers provide e-mail alerts. Others feature a Web-based approach where alarms and messages are transmitted first to central servers, and then to dispatched to the appropriate maintenance personnel. Auto dialers that can be used with Twitter are also available.