Interfacing the Serial/RS-232 Port

Interfacing the Serial / RS232 Port V5.0
Note: The source code on the earier pages is not a really good example on how to
program but is rather cut down to size giving quick results, and making it easier
to understand. Upon executing your communications program, it would be wise
to store the status of the UART registers, so that they all can be restored before
you quit the program. This is to cause the least upset to other programs which
may also be trying to use the communications ports.
The first step to using interrupts is to work out which interrupt services your serial card. Table 13 shows
the base addresses and IRQ's of some standard ports. IRQ's 3 and 4 are the two most commonly used.
IRQ 5 and 7 are sometimes used.
Interrupt Vectors
Once we know the IRQ the next step is to find it's interrupt vector or software interrupt as some
people may call it. Basically any 8086 processor has a set of 256 interrupt vectors numbered 0 to 255.
Each of these vectors contains a 4 byte code which is an address of the Interrupt Service Routine (ISR).
Fortunately C being a high level language, takes care of the addresses for us. All we have to know is the
actual interrupt vector.
INT (Hex) IRQ Common Uses
08 0 System Timer
09 1 Keyboard
0A 2 Redirected
0B 3 Serial Comms. COM2/COM4
0C 4 Serial Comms. COM1/COM3
0D 5 Reserved/Sound Card
0E 6 Floppy Disk Controller
0F 7 Parallel Comms.
70 8 Real Time Clock
71 9 Reserved
72 10 Reserved
73 11 Reserved
74 12 PS/2 Mouse
Interfacing the Serial / RS232 Port V5.0 Page 27