In order to communication over serial lines such as telephone lines we need to connect the microprocessor to a USART. The microprocessor sends parallel 8 bit data to the USART. The USART latches the data byte into its register and inserts start bit, parity bit and one or more stop bits. This character frame is then serialized using Parallel to serial converter and send out in serially by the performing shifting operation. The signal coming out of the USART is TTL signals which has not enough strength to be send over long distance. So this TTL signal is converted to the higher voltage by voltage converter. MAX232 is a device that converts TTL voltages levels(0 to 0.8V for logic 0 and 2V to Vcc for logic 1) to RS232 voltage levels(-3 to -15V for logic 0 and +3 to +15V for logic 1) and from RS232 voltage levels to TTL voltage levels. The output from the MAX232 are thus RS232 signals and connected to a male DB25 connector or 9 DB connector. The DB25 or DB9 connector are connected to the female DB25 or female 9DB connected. The DB25 female/DB9 female connector are connected to the MODEM which is a DCE(Data Communication Equipment).
The following schematic shows how a 8086 microprocessor is interfaced with intel 8251 USART which is in turn connected to the MAX232 transceiver and finally to the DB25 connector.
The overview of the connection was explained previously. Now a more detailed explanation will follow. Looking at the 8251 chip the important pins or signals are the DTR, DSR, RTS and CTS. These are all control signals. The DTR and DSR signals are used to initialize the DTE(microprocessor or CPU or Computer) and the modem. The RTS and CTS are control signals which are inserted before data transfer. The MAX232 has two drivers and two receivers. In the figure above the TxD and RxD are the pins over which the serial data is sent. These are connected to the T1IN and R1OUT respectively. Similarly the RTS and CTS signals are connected to the T2OUT and R2IN.
The following schematic shows how a 8086 microprocessor is interfaced with intel 8251 USART which is in turn connected to the MAX232 transceiver and finally to the DB25 connector.
The overview of the connection was explained previously. Now a more detailed explanation will follow. Looking at the 8251 chip the important pins or signals are the DTR, DSR, RTS and CTS. These are all control signals. The DTR and DSR signals are used to initialize the DTE(microprocessor or CPU or Computer) and the modem. The RTS and CTS are control signals which are inserted before data transfer. The MAX232 has two drivers and two receivers. In the figure above the TxD and RxD are the pins over which the serial data is sent. These are connected to the T1IN and R1OUT respectively. Similarly the RTS and CTS signals are connected to the T2OUT and R2IN.
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