Currently PLC remote control has become an alternative to efficiency in manufacturing. Today the developing of supervisory control is a result of developments in communications technology and electronic devices that provide high level of reliability in control.
This interface modem to the PLC is a device that lets it communicate with the PLC with modem. This tool provides remote access to PLC control with easy access from various places, reliable and has a cheaper cost.
The general working of the modem interface is as follows. To remote the PLC is controlled by Personal Computer (PC) via the first modem, and so on through the phone line (PSTN). From a phone line, the data will be entered into the modem 2 and then will be forwarded to the PLC via a modem interface.
Meanwhile, if the PCs want to take the data from the PLC, then the instruction from the PC interface will be transmitted by modem and after getting the data will be sent back to the PC.
Baud rate used for the systems are 9600 bps, since the PLC is generally wore the baud rate. While the modem is used for modem U.S. Robotics 56K for PC and modem D-Link 56K for modem interface.
Modem hardware interface is the minimum system which uses MCS-51 microcontroller family, which is 80C31BH. This microcontroller has a single serial communication channel (serial port), it requires a serial port interface, one for communication with the modem and one for communication with the PLC.
For the second serial port, use the Universal Asynchronous Receiver / Transmitter (UART) SCC2691 type controller from Philips. While the external memory used to program EPROM 16 Kbytes 27128th capacity, and to use external RAM data memory with a capacity of 32 Kbytes 62256th.