Serial port is a way of communication among two devices just like the
parallel port. The basic difference is that whole bytes are sent from one place
to another in case of parallel port while the bits are sent one by one on the
serial port in a specially formatted fashion. The serial port connection is a
9pin DB-9 connector with pins assigned as shown below.
We have made a wire that connects signal ground of the two connectors,
the TD of one to the RD of the other and the RD of one to the TD of the other.
This three wire connection is sufficient for full duplex serial communication.
The data on the serial port is sent in a standard format called RS232
communication. The data starts with a 1 bit called the start bit, then five to
eight data bits, an optional parity bit, and one to two 0 bits called stop bits.
The number of data bits, parity bits, and the number of stop bits have to be
configured at both ends. Also the duration of a bit must be precisely known
at both ends called the baud rate of the communication.
The BIOS INT 14 provides serial port services. We will use a mix of BIOS
services and direct port access for our example. A major limitation in using
BIOS is that it does not allows interrupt driven data transfer, i.e. we are
interrupted whenever a byte is ready to be read or a byte can be transferred
since the previous transmission has completed. To achieve this we have to
resort to direct port access. Important BIOS services regarding the serial port
are discussed below.
INT 14 - SERIAL - INITIALIZE PORT
AH = 00h
AL = port parameters
DX = port number (00h-03h)
Return:
AH = line status
AL = modem status
Every bit of line status conveys different information. From most
significant to least significant, the meanings are timeout, transmitter shift
register empty, transmitter holding register empty, break detect, receiver
ready, overrun, parity error, and framing error. Modem status is not used in
direct serial communication. The port parameters in AL consist of the baud
1โCarrier Detect
(CD)
2โ Received Data
(RD)
3โTransmitted
Data (TD)
4โData Terminal
Ready
(DTR)
5โ Signal Ground
6โ Data Set
Ready
(DSR)
7โ Request to
Send (RTS)
8โ Clear to Send
(CTS)
9โ Ring Indicator
(RI)
