The MAX232 requires 5 external 10uF capacitors. These are used by the internal charge pump to create +10 volts and -10 volts. The MAX232 includes 2 receivers and 2 transmitters so two serial ports can be used with a single chip. We will only use one transmitter for this project. The only connection that must be made to the 8052 is one jumper from pin 3 of the 8052 to pin 11 of the MAX232.
The circuit diagram shown below illustrates the connection of RS232 with microcontroller and serial port DB9 connector.Data is transmitted and received on pins 2 and 3 respectively. Data Set Ready (DSR) is an indication from the Data Set (i.e., the modem or DSU/CSU) that it is on. Similarly, DTR indicates to the Data Set that the DTE is on. Data Carrier Detect (DCD) indicates that a good carrier is being received from the remote modem.
Pins 4 RTS (Request To Send - from the transmitting computer) and 5 CTS (Clear To Send - from the Data set) are used to control. In most Asynchronous situations, RTS and CTS are constantly on throughout the communication session. However where the DTE is connected to a multipoint line, RTS is used to turn carrier on the modem on and off. On a multipoint line, it's imperative that only one station is transmitting at a time (because they share the return phone pair). When a station wants to transmit, it raises RTS. The modem turns on carrier, typically waits a few milliseconds for carrier to stabilize, and then raises CTS. The DTE transmits when it sees CTS up. When the station has finished its transmission, it drops RTS and the modem drops CTS and carrier together.
Clock signals (pins 15, 17, & 24) are only used for synchronous communications. The modem or DSU extracts the clock from the data stream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals do not have to be the same, or even at the same baud rate.
Note: Transmit and receive leads (2 or 3) can be reversed depending on the use of the equipment - DCE Data Communications Equipment or a DTE Data Terminal Equipment.
CTS Clear To Send
DCD Data Carrier Detected (Tone from a modem)
DCE Data Communications Equipment eg. modem
DSR Data Set Ready
DSRS Data Signal Rate Selector (Not commonly used)
DTE Data Terminal Equipment eg. computer, printer
DTR Data Terminal Ready
FG Frame Ground (screen or chassis)
NC No Connection
RCk Receiver (external) Clock input
RI Ring Indicator (ringing tone detected)
RTS Request To Send
RxD Received Data
SG Signal Ground
SCTS Secondary Clear To Send
SDCD Secondary Data Carrier Detected (Tone from a modem)
SRTS Secondary Request To Send
SRxD Secondary Received Data
STxD Secondary Transmitted Data
TxD Transmitted Data
The RS232 connector was originally developed to use 25 pins. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel.
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