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Serial Port IO
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The javax.comm serial port classes assume the system has one or more ports following the RS-232 (or EIA232) standard. The standard originated in the 1960s and dealt with a computer talking to a display terminal. This explains some of the names describing the 9 pins for the DB9 connector in Table 22.1. (See the references for a description of the less common 25-pin DB25 connector.)

Serial ports send and receive one bit at a time using a positive voltage (between 3 and 25V) to indicate a 0 bit and a negative voltage (between -3 to -25V) to indicate a 1 bit. The duration of a voltage depends inversely on the baud rate.

An asynchronous serial protocol (not included in the RS-232 standard) is required to determine how to decode the bits into bytes. The standard protocol groups the bits into a standard data unit (SDU) consisting typically of 7 data bits (for ASCII) or 8 data bits. To indicate where a SDU begins, a start bit value of value 0 is sent. To indicate the end of a SDU, the group ends with one or two stop bits each of value 1. To combat noise and bit errors, a parity bit is usually included. For even parity the bit value is 1 if the number of one bit in the SDU is even.  For odd parity the bit value is 1 if the number of one bit is odd. So, depending on the protocol settings, the total number of bits sent for an SDU can vary from 9 bits (7 data bits, 1 stop bit and no parity) up to 12 bits (8 data bits, 2 stop bits and a parity bit.)

The SerialPort class provides a single method to set the baud rate, number of data bits, number of stop bits, and the parity:

 void SetSerialPortParams (int baud, int dataBits, int stopBits, int parity)

If a port does not support any of the values passed in the parameters, the method will throw the UnsupportedCommOperationException. 

The SerialPort class includes a set of constants to use for these parameters such as

DATABITS_7
DATABITS_8
STOPBITS_1
STOPBITS_2
PARITY_NONE
PARITY_EVEN
PARITY_ODD

The two devices connected via the serial line need flow control settings to determine who is sending and who is receiving and when to switch between the two states. The XON/XOFF and RTS/CTS are the two primary protocols for this.  The former is a software protocol while the latter is implemented in hardware. The methods

  void setFlowControlMode (int protocol)
  int getFlowControlMode ()

provide for setting and getting the flow control mode. The protocol value sets both input and output protocols with a bitwise AND of the constants in the SerialPort class:

FLOWCONTROL_NONE
FLOWCONTROL_XONXOFF_IN
FLOWCONTROL_XONXOFF_OUT
FLOWCONTROL_RTSCTS_IN
FLOWCONTROL_RTSCTS_OUT

The set method will throw the UnsupportedCommOperationException if the protocol value is invalid.

The table shows the pin assignments for an RS-232 9-pin connector on a computer. The Function column indicates the purpose of each pin for the case of a computer connected to a modem. The serial line sends and receives only one bit at a time but it uses separate lines for transmission and reception and it uses six other lines (not counting the ground line) for setting up the protocol for the communications.

Serial Line Connections

The javax.comm class CommPort provides methods to access the six control lines. For the two output control wires, DTR and RTS, there are methods both to set the line and to find the current setting:

void setDTR (boolean val)
boolean isDTR ()
void setRTS (Boolean val)
boolean isRTS ()

For the other four input control lines, there are  methods to find their current state:

boolean isCTS ()
boolean isDSR ()
boolean isRI ()
boolean isCD ()

For connecting to devices other than modems, it is often unnecessary to use all of these  control lines. In the serial line demonstration program below, for example, we only the CTS and RTS (plus the ground line) lines are active.  

Latest update: Dec. 13, 2004